Genetics Final Flashcards

Question

Describe the contractile rings with cytokinesis

Answer 1

A contractile ring of actin filaments forms at the equator of the cell, beneath the plasma membrane. As this ring contracts, it pulls the membrane inward, creating a cleavage furrow that eventually divides the cell into two genetically identical daughter cells. The process is guided by signals from the spindle midzone and is tightly regulated to ensure the proper partitioning of the cytoplasm and organelles.

Answer 2

germ cells meiosis I meiosis II

Answer 3

Prophase I

Answer 4

1. Leptotene: Chromosomes start to condense. 2. Zygotene: Homologous chromosomes pair up (synapsis). 3. Pachytene: Crossing-over happens between homologous chromosomes. 4. Diplotene: Homologous chromosomes start to separate but remain connected at chiasmata. 5. Diakinesis: Chromosomes further condense, preparing for the next stage of meiosis

Answer 5

Homologous chromosome pairs align at the equator of the cell (the metaphase plate). Each homolog is attached to spindle microtubules from opposite poles, ensuring that they will be pulled apart in the next phase.

Answer 6

The homologous chromosomes are pulled apart toward opposite poles of the cell. This is the key step in reductional division, where the chromosome number is halved. Each resulting daughter cell will have one copy of each homologous chromosome.

Answer 7

The separated chromosomes arrive at the poles, and the cell may undergo cytokinesis to form two daughter cells. These cells are haploid, but each chromosome still consists of two sister chromatids. This is followed by a short resting phase called interkinesis, without further DNA replication.

Answer 8

This phase is similar to mitosis and is known as equational division, where the sister chromatids are separated. - Prophase II: Chromosomes condense, and a new spindle apparatus forms. - Metaphase II: Chromosomes line up at the metaphase plate, with each sister chromatid attached to microtubules from opposite poles. - Anaphase II: Sister chromatids are pulled apart and move toward opposite poles. - Telophase II: Chromatids reach the poles, and nuclear membranes reform around each set of chromosomes. Cytokinesis follows, resulting in four haploid gametes.

Answer 9

nondisjunction aneuploidies

Answer 10

During metaphase I, homologous chromosome pairs align randomly at the metaphase plate. Each pair can orient itself independently of the other pairs, meaning that either the maternal or paternal homolog can be pulled to a given pole during anaphase I. This random distribution of homologs generates different combinations of chromosomes in the gametes, following Mendel’s Law of Independent Assortment.

Answer 11

In prophase I, homologous chromosomes exchange genetic material at points of crossing-over (chiasmata). This process shuffles alleles between homologous chromosomes, creating new combinations of genes. As a result, sister chromatids are no longer identical, and when chromatids are separated during anaphase II, each gamete will carry a unique set of genetic material. This genetic recombination contributes significantly to genetic diversity in offspring.

Answer 12

- Prophase I: Chromosomes condense, homologs undergo synapsis and form tetrads, and crossing-over occurs at chiasmata. - Metaphase I: Homologous chromosome pairs align at the metaphase plate. - Anaphase I: Homologs are pulled to opposite poles, reducing the chromosome number by half. - Telophase I: Two haploid cells are formed, though each chromosome still consists of two sister chromatids

Answer 13

- Prophase II: Chromosomes condense again in both daughter cells. - Metaphase II: Chromosomes align at the metaphase plate, with sister chromatids facing opposite poles. - Anaphase II: Sister chromatids separate and move to opposite poles. - Telophase II: Four haploid gametes are formed after cytokinesis.

Answer 14

This law states that during the formation of gametes, the two alleles for each gene segregate so that each gamete carries only one allele for each gene. During anaphase I of meiosis, homologous chromosomes are separated, ensuring that each gamete receives only one copy of each chromosome (and hence, one allele of each gene).

Answer 15

A typical male has one X chromosome and one Y chromosome. The SRY gene on the Y chromosome initiates the development of male characteristics, making the individual male.

Answer 16

A typical female has two X chromosomes. Since there is no Y chromosome (and thus no SRY gene), the individual develops as female.

Answer 17

This is a male with an extra X chromosome. Despite the extra X, the presence of the Y chromosome with the SRY gene leads to male development, although affected individuals may have some atypical male characteristics and may be infertile.

Answer 18

Individuals with only one X chromosome (and no Y chromosome) develop as females. They often have underdeveloped female characteristics and may have health issues related to missing one X chromosome.

Answer 19

This karyotype results in a male because of the presence of a Y chromosome with the SRY gene. These individuals tend to be phenotypically normal males.

Answer 20

This results in a female, as there is no Y chromosome. These individuals generally develop as females, though they may be taller than average and have some mild learning difficulties.

Answer 21

In some cases, individuals with an XY karyotype develop as females. This can happen if the SRY gene is deleted or non-functional, meaning the signals for male development are absent. Without the action of SRY, the gonads develop into ovaries, leading to a female phenotype despite the presence of a Y chromosome.

Answer 22

In this case, individuals with two X chromosomes develop as males. This occurs if the SRY gene is translocated onto one of the X chromosomes during meiotic recombination. The presence of SRY triggers male development despite the absence of a Y chromosome.

Answer 23

downstream

Answer 24

Birds have a ZZ/ZW system, where males are the homogametic sex (ZZ) and females are the heterogametic sex (ZW). Here, it is the female that determines the sex of the offspring. Unlike mammals, there is no SRY gene involved in determining sex, and the molecular basis of sex determination in birds involves different genes such as DMRT1.

Answer 25

Humans follow the XX/XY system of sex determination. Males are the heterogametic sex, producing sperm with either an X or Y chromosome, while females are the homogametic sex, producing eggs with only X chromosomes. The presence of a Y chromosome, specifically the SRY gene, directs male development. The pseudoautosomal regions (PARs) of the X and Y chromosomes allow them to pair and recombine during meiosis.

Answer 26

In fruit flies (Drosophila), sex is determined by the ratio of X chromosomes to autosomes rather than the presence of a Y chromosome. A ratio of 1:1 (two X chromosomes for two sets of autosomes) results in a female, while a ratio of 0.5 (one X chromosome for two sets of autosomes) results in a male. The Y chromosome in flies is not necessary for male development, but it is required for fertility.

Answer 27

In many reptiles, such as turtles and crocodiles, sex is determined by the incubation temperature of the eggs rather than chromosomes. Higher temperatures may result in one sex (e.g., females), while lower temperatures result in the other sex. There is no involvement of X or Y chromosomes in these cases.

Answer 28

In hymenopterans like bees and ants, sex is determined by ploidy. Females are diploid (having two sets of chromosomes), while males are haploid (having one set of chromosomes). Males develop from unfertilised eggs, while females develop from fertilised eggs. There are no X or Y chromosomes involved in this system.

Answer 29

Heterogametic Homogametic

Answer 30

Location: Occurs in the ovaries. Cells: Starts with oogonia (female germ cells). Timeline: * Begins in fetal development. * Arrested at prophase I until puberty. * One secondary oocyte produced per menstrual cycle. Meiosis: * Resumes at ovulation, completes Meiosis II only if fertilised. Products: * One ovum and 2–3 polar bodies (degenerate byproducts). Cytoplasm: * Unequal division—most goes to the secondary oocyte. Lifespan: * ~400,000 oocytes at birth; ~400 ovulate during the reproductive years. Long process: Starts in utero, finishes only upon fertilisation.

Answer 31

ovum polar bodies four

Answer 32

fetal puberty

Answer 33

- Location: Occurs in the testes. - Cells: Begins with spermatogonia (male germ cells). - Timeline: * Starts at puberty, continues throughout life. * Primary spermatocytes enter meiosis at puberty and develop continuously. - Meiosis: * Completed before sperm maturation, producing mature sperm regularly. - Products: * Four sperm cells per spermatogonium. * No polar bodies. - Cytoplasm: * Equal division—results in four spermatids of similar size. - Lifespan: * Virtually infinite—hundreds of millions of sperm produced daily. - Process: Takes 64–72 days to complete and occurs continuously throughout a male's reproductive life

Answer 34

unequal equal

Answer 35

Oogonia are the female germ cells in the ovaries that give rise to oocytes, or egg cells. They are diploid cells (2n) formed during fetal development and undergo mitosis to increase in number. As development progresses, some oogonia differentiate into primary oocytes, which enter prophase I of meiosis and become arrested in this stage until puberty. Oogonia play a crucial role in female reproductive development, as they are the precursors to the eggs needed for fertilization.

Answer 36

A primary oocyte is a diploid (2n) cell that forms from oogonia during female fetal development. It is characterized by being arrested in prophase I of meiosis, where it remains until puberty. Each primary oocyte is surrounded by a layer of granulosa cells, forming a structure known as a primordial follicle. Upon maturation during the menstrual cycle, primary oocytes complete meiosis I, resulting in a secondary oocyte and a polar body, which is typically discarded. Primary oocytes are essential for female fertility, as they ultimately develop into the eggs that can be fertilized by sperm.

Answer 37

A secondary oocyte is a haploid (n) cell that results from the completion of meiosis I by a primary oocyte. During this process, the primary oocyte divides asymmetrically, producing one large secondary oocyte and a smaller polar body, which usually degenerates. The secondary oocyte enters meiosis II but is arrested in metaphase II until fertilization occurs. If fertilization takes place, the secondary oocyte will complete meiosis II, resulting in a mature ovum and another polar body. The secondary oocyte is the cell that is released during ovulation and is capable of being fertilized by sperm.

Answer 38

- Autosome complement: 22 autosomes Sex chromosome complement: X - Stage: The mature egg cell, formed after fertilisation, following the completion of meiosis II. The second polar body is also formed during this stage.

Answer 39

- Autosome complement: 44 autosomes - Sex chromosome complement: XY - Stage: Diploid germ cells in the testes, undergo mitosis to produce primary spermatocytes.

Answer 40

- Autosome complement: 44 autosomes - Sex chromosome complement: XY - Stage: Undergoes meiosis I to produce secondary spermatocytes.

Answer 41

- Autosome complement: 22 autosomes - Sex chromosome complement: X or Y - Stage: Formed after the first meiotic division. Each secondary spermatocyte contains either an X or a Y chromosome, along with 22 autosomes. Undergoes meiosis II.

Answer 42

- Autosome complement: 22 autosomes - Sex chromosome complement: X or Y - Stage: Formed after the second meiotic division. These haploid cells develop into mature sperm.

Answer 43

- Autosome complement: 22 autosomes - Sex chromosome complement: X or Y - Stage: The final, mature male gametes. Each sperm will carry either an X or a Y chromosome, determining the sex of the future offspring.

Answer 44

sex-linked

Answer 45

Thomas Hunt Morgan’s crisscross inheritance

Answer 46

For example, when Morgan crossed white-eyed male flies (a recessive X-linked trait) with red-eyed females (dominant), he found that all female offspring had red eyes and all male offspring had white eyes in the F1 generation. In the F2 generation, females could carry the recessive allele and pass it to their sons, which created the characteristic crisscross inheritance

Answer 47

Crisscross inheritance occurs when a trait is passed from an affected male to his daughters, who are carriers (if heterozygous), and from those daughters to their sons. This pattern suggests X-linked inheritance.

Answer 48

Hemizygous males are individuals, typically referring to male organisms with one X chromosome and one Y chromosome (such as in humans and many other species). Because they have only one copy of the X chromosome, any allele present on that chromosome will be expressed, whether it is dominant or recessive. This is in contrast to females, who have two X chromosomes and can be homozygous or heterozygous for X-linked traits. Hemizygosity makes males particularly susceptible to expressing X-linked recessive traits, such as color blindness or hemophilia, since they lack a second X chromosome that could potentially mask the recessive allele.

Answer 49

1. The trait predominantly affects males in the family. 2. Male-to-male transmission does not occur (since males pass their Y chromosome, not the X chromosome, to their sons). 3. There is a crisscross inheritance pattern with daughters acting as carriers, passing the trait to their sons.

Answer 50

Morgan’s work provided the first experimental evidence that genes reside on chromosomes, by showing that the inheritance of certain traits is linked to the inheritance of specific chromosomes. His experiments with sex-linked traits demonstrated that chromosomes are not just carriers of genetic material but also the location of individual genes.

Answer 51

nondisjunction For example, when nondisjunction occurred, offspring with abnormal numbers of X or Y chromosomes (e.g., XXY or X0) displayed phenotypes that correlated with the inheritance of extra or missing sex chromosomes, proving that chromosomes carry the genetic instructions for trait expression.

Answer 52

sex linkage nondisjunction

Answer 53

- Males (hemizygous) express any allele on their single X chromosome. - Affected males inherit the trait from carrier mothers (heterozygous). - Females express the trait only with two recessive alleles (one from each parent). - Affected males cannot pass the X chromosome to sons (fathers pass Y). - Affected males have unaffected daughters (carriers) who can pass the trait to their sons.

Answer 54

1. Identify Affected Individuals: Start by marking all affected individuals (usually males) in the pedigree. 2. Trace Maternal Lines: Look for affected males to trace back to their mothers, identifying carrier mothers. 3. Check Female Progeny: Examine daughters of affected males; if they are unaffected, they may still be carriers. 4. Evaluate Sons of Carriers: Assess the next generation by tracking sons of carrier females for the expression of the trait. 5. Look for Patterns: Identify crisscross patterns of inheritance and absence of male-to-male transmission.

Answer 55

- Both males and females can be affected, but females may show milder symptoms because of X-chromosome inactivation (more on this below). - Affected males pass the trait to all of their daughters (since daughters inherit their father’s X chromosome) but none of their sons. - Affected females pass the trait to about half of their children, regardless of sex. - Affected females may show more variability in the severity of symptoms due to random inactivation of one X chromosome.

Answer 56

Identify Affected Individuals: - Mark all individuals exhibiting the trait (typically shaded shapes in the pedigree). Determine Gender of Affected Individuals: - Note that both males and females can be affected, but affected males will have affected daughters. Look for Male-to-Female Transmission: - Affected males will pass the trait to all of their daughters (but not to their sons). Assess Female Progeny: - Affected females can pass the trait to about 50% of their offspring (both sons and daughters). Look for affected children of affected mothers. Check for Unaffected Parents: - Unaffected individuals who have affected children may indicate that the trait is inherited from a carrier mother, especially if the mother is not affected herself. Examine Generational Patterns: - Observe that the trait typically appears in every generation due to the dominant nature of the allele. There should be no skipping of generations. Consider Incomplete Penetrance: - Be aware that some individuals may carry the allele but not express the trait visibly. Look for Carrier Females: - If a female is affected and has an unaffected child, the child may be a carrier. Analyze for Crisscross Patterns: - Unlike X-linked recessive traits, X-linked dominant traits do not exhibit the same crisscross inheritance pattern. Instead, affected males will directly affect their daughters.

Answer 57

- Both males and females are equally likely to be affected. - Affected individuals typically have unaffected parents who are carriers (heterozygous). - The trait may appear to "skip" generations as it reappears when two carriers have children.

Answer 58

- Both males and females are equally likely to be affected. - The trait usually appears in every generation, with affected individuals passing it on to about half of their offspring, regardless of sex. - Male-to-male transmission can occur, which helps distinguish autosomal dominant traits from X-linked traits.

Answer 59

- Only males are affected, since only males inherit the Y chromosome. - The trait is passed from an affected father to _all of his sons._ - There is no female involvement in the transmission of Y-linked traits.

Answer 60

- Females have two X chromosomes (XX), while males have only one (XY). If both X chromosomes in females were fully active, it would result in dosage imbalance—females would produce twice as many X-linked gene products as males. To prevent this imbalance, dosage compensation mechanisms exist to equalise gene expression between the sexes. - A main mechanism of this is _x-inactivation_

Answer 61

- In females, one of the two X chromosomes in each cell is randomly inactivated early in development. This process, called X-chromosome inactivation, ensures that females, like males, have only one functional X chromosome per cell. - The inactivated X chromosome becomes a dense, inactive structure known as a Barr body. Barr bodies are visible in the nucleus and can be used to determine the number of inactivated X chromosomes. - X-inactivation occurs randomly in each cell, so some cells in a female express genes from one X chromosome, and other cells express genes from the other X chromosome. This creates a mosaic pattern of gene expression in females.

Answer 62

- The process is initiated in a region of the X chromosome known as the X-inactivation centre (XIC). - A key gene in this region, XIST, produces an RNA transcript that coats the X chromosome to be inactivated, leading to its silencing. - Once inactivated, the X chromosome remains inactive in all descendant cells, creating a clone of cells with the same X chromosome inactivated. This results in tissue mosaicism for X-linked genes.

Answer 63

traits that are expressed in only one sex, despite being determined by autosomal or X-linked genes. For example, milk production in mammals is a sex-limited trait that is only expressed in females, even though both sexes carry the genes for it.

Answer 64

Sex-influenced traits are autosomal traits where the expression of the trait differs between males and females due to hormonal differences. For instance, male-pattern baldness is a sex-influenced trait where the dominant allele results in baldness in males but has little effect in females unless they are homozygous for the trait.

Answer 65

- Bipotential gonads (genital ridge) - Mullerian ducts - Wolffian ducts - External genitalia (cloaca)

Answer 66

- Initially, all embryos have bipotential gonads, meaning these structures can develop into either testes or ovaries depending on genetic and hormonal cues. - In XY individuals, the SRY gene (Sex-determining Region Y) on the Y chromosome triggers the gonads to differentiate into testes around week 6-7 of development. In the absence of the SRY gene (in XX individuals), the gonads develop into ovaries.

Answer 67

- These ducts are precursors to the female internal reproductive organs, including the fallopian tubes, uterus, and upper part of the vagina. - In XY individuals, the testes produce Anti-Müllerian Factor (AMF), a hormone that causes the Müllerian ducts to regress, preventing the development of female reproductive structures. - In XX individuals, without AMF, the Müllerian ducts develop into the female internal reproductive organs.

Answer 68

- These ducts are the precursors to the male internal reproductive organs, including the epididymis, vas deferens, and seminal vesicles. - In XY individuals, testosterone produced by the testes stimulates the development of the Wolffian ducts into these male reproductive structures. - In XX individuals, the lack of testosterone causes the Wolffian ducts to degenerate.

Answer 69

- The external genitalia initially develop from a common structure called the cloaca, which later differentiates into either male or female genitalia. - In XY individuals, DHT (dihydrotestosterone), a more potent form of testosterone, is critical for the masculinisation of the external genitalia, forming the penis and scrotum. - In XX individuals, the absence of DHT leads to the development of female external genitalia, including the clitoris and labia.

Answer 70

In XY individuals, mutations in genes involved in sex differentiation can lead to conditions where individuals have both male and female characteristics or ambiguous genitalia. This can result in intersexuality, where the typical pathways for male development are disrupted.

Answer 71

- Mutations in the androgen receptor (AR) gene result in cells' inability to respond to androgens like testosterone and DHT. - XY individuals with a functional SRY gene produce normal testosterone and DHT levels, but their bodies cannot utilize these hormones. - The Wolffian ducts do not develop into male internal reproductive organs, and external genitalia follow a female pathway, leading to typically female external genitalia. - However, they lack a uterus and fallopian tubes due to Anti-Müllerian Factor (AMF) causing Müllerian duct regression. - Individuals with CAIS are genetically male (XY) but phenotypically female, often remaining undiagnosed until puberty when menstruation fails to occur.

Answer 72

- In this condition, there is a mutation in the gene encoding the enzyme 5-alpha-reductase, which converts testosterone into DHT. - DHT is necessary for the masculinisation of the external genitalia in XY individuals. Without it, individuals may be born with ambiguous genitalia or female-appearing external genitalia. - However, at puberty, when testosterone levels rise, some masculinisation may occur, leading to the development of more male-typical characteristics like increased muscle mass, voice deepening, and growth of facial hair. - These individuals have testes and may develop male internal reproductive organs, but the lack of DHT during early development can result in ambiguous genitalia at birth.

Answer 73

- If there is a mutation in the SRY gene, the bipotential gonads may fail to develop into testes, despite the presence of an XY karyotype. - This can lead to an individual developing female-typical internal and external genitalia, even though they are genetically male. - This form of sex reversal occurs because, without SRY function, the typical male differentiation pathway is not initiated.

Answer 74

- In PAIS, mutations in the androgen receptor lead to partial responsiveness to androgens. - The degree of intersexuality can vary depending on how much androgen signalling is disrupted. Individuals may have ambiguous genitalia or predominantly male or female genitalia with atypical features. - Testosterone may stimulate partial development of the male reproductive structures, but without full androgen signalling, the individual may not fully masculinise.

Answer 75

- Refers to the situation where two or more genes are located close together on the same chromosome, leading to their tendency to be _inherited together during meiosis._ This happens because genes that are physically close to each other are less likely to be separated by the process of crossing over during recombination. Therefore, linked genes _do not follow Mendel's law of independent assortment_ and tend to be passed down as a set

Answer 76

Parental recombinant Recombinants

Answer 77

9:3:3:1 parental recombinant

Answer 78

- Crossing a heterozygote with a homozygous recessive individual - Useful for detecting linkage - If the genes are unlinked, the testcross would produce equal numbers of the four possible phenotypes (1:1:1:1). However, if the genes are linked, the result will be skewed, with more offspring showing parental-type combinations and fewer showing recombinant combinations. The proportion of recombinant offspring reflects the distance between the genes, with _fewer recombinants indicating tighter linkage._

Answer 79

- A genetic process that occurs during prophase I of meiosis, where homologous chromosomes exchange segments of DNA. This reciprocal exchange of genetic material results in new allele combinations, contributing to genetic diversity in offspring. The sites of crossing over are called chiasmata, and the process plays a crucial role in the proper segregation of chromosomes during meiosis.

Answer 80

- Crucial for the proper segregation of homologous chromosomes during meiosis I. They are the physical manifestations of crossing over and help hold homologous chromosomes together after they synapse (pair). During anaphase I, the tension at the chiasmata ensures that the homologous chromosomes are pulled apart correctly towards opposite poles, reducing the risk of chromosomal abnormalities such as nondisjunction.

Answer 81

The recombination frequency (RF) is the proportion of recombinant offspring compared to the total offspring in a genetic cross, and it reflects the physical distance between two loci on a chromosome. The further apart two loci are, the more likely a crossover event will occur between them, resulting in a higher RF

Answer 82

Map distance is measured in centiMorgans (cM), where 1 cM equals a 1% recombination frequency. A higher RF indicates that the loci are further apart, while a lower RF suggests they are closely linked.

Answer 83

The maximum recombination frequency (RF) between any two genes is limited to 50% due to the nature of gene linkage and independent assortment. When genes are unlinked (located on different chromosomes or far apart on the same chromosome), they assort independently, leading to an RF close to 50%. However, for linked genes, even with maximal recombination events (double crossover events, DCO), the proportion of recombinant gametes cannot exceed 50%, as there will always be a significant number of non-crossover (NCO) or single crossover (SCO) gametes produced. This limitation ensures that any genetic mapping using RF remains valid

Answer 84

A two-point testcross involves crossing an individual heterozygous for two genes (e.g., AaBb) with a homozygous recessive individual (aabb). The offspring can display either parental phenotypes (matching the parents) or recombinant phenotypes (new combinations resulting from crossing over).

Answer 85

The proportion of recombinant offspring reveals the recombination frequency (RF) between the two loci, which is a measure of how frequently crossing over occurs between them. For example, if 10% of the offspring are recombinants, the two genes are 10 map units (centiMorgans) apart. This distance allows us to position genes relative to each other on a chromosome. The smaller the RF, the closer the genes are; the larger the RF, the further apart they are.

Answer 86

A testcross involves crossing an individual exhibiting a dominant phenotype (whose genotype is unknown) with a homozygous recessive individual. If the unknown parent is homozygous for the dominant alleles of two linked genes, the offspring will show predominantly parental phenotypes, with few or no recombinants. Conversely, if the unknown parent is heterozygous and the genes are unlinked, we would expect a 1:1:1:1 ratio of phenotypes among the progeny, indicative of independent assortment. Therefore, the presence of a preponderance of parental classes of gametes in the testcross progeny supports the hypothesis of linkage, while a more even distribution of phenotypes suggests the genes are assorting independently.

Answer 87

Synaptonemal complexes are protein structures that form between homologous chromosomes during prophase I of meiosis, facilitating their pairing and alignment. These complexes help stabilize the connection between homologs, ensuring accurate recombination and crossing over at chiasmata. By promoting close interaction between chromosomes, synaptonemal complexes play a crucial role in the proper segregation of chromosomes during meiosis

Answer 88

Reciprocal exchanges refer to the process during meiosis where segments of DNA are exchanged between non-sister chromatids of homologous chromosomes. This exchange occurs during crossing over, leading to new combinations of alleles on the chromosomes. As a result, reciprocal exchanges enhance genetic diversity in gametes, contributing to the variation seen in offspring

Answer 89

- Non-crossover (NCO) configurations refer to gametes that retain the original combinations of alleles from the parents, showing no recombination. - Single crossover (SCO) configurations result from one crossover event between homologous chromosomes, producing gametes with new allele combinations for the genes involved. - Double crossover (DCO) configurations occur when two separate crossover events happen between the same homologous chromosomes, leading to a more complex combination of alleles, which may restore some parental combinations while introducing new ones for the intervening genes.

Answer 90

- By performing multiple two-point crosses, the RF between pairs of loci can be compared. However, this approach has limitations, such as not accounting for double crossovers (DCOs), which can skew the results and underestimate the distance between genes.

Answer 91

Three-point testcrosses involve a parent heterozygous for three loci crossed with a homozygous recessive individual. This approach provides more accurate mapping because it allows for the identification of the gene in the middle. To analyze the data, researchers count the offspring phenotypes and calculate RFs for the pairs of loci. The formula for calculating RF for three loci involves considering single crossovers (SCOs) and double crossovers (DCOs).

Answer 92

By determining which phenotypic classes are less frequent, one can identify the gene that is in the middle. The correction for double crossovers is essential, as these events can confound the estimated distances. This refinement results in a more accurate genetic map and often reveals nonuniform crossover frequencies among different gene pairs.

Answer 93

Nonuniform crossover frequencies refer to the variation in the likelihood of crossover events occurring at different regions along a chromosome during meiosis. Certain areas, often influenced by chromatin structure and gene density, may experience higher or lower rates of recombination. This variation can lead to uneven genetic mapping distances and affect the accuracy of genetic maps, as some loci may be more likely to undergo crossover than others.

Answer 94

Genetic maps, measured in map units (m.u.) or centimorgans (cM), represent relative distances based on recombination frequencies. While 1% RF corresponds to 1 m.u., the relationship between genetic map distances and actual physical distances (in base pairs) is not linear. This discrepancy arises from nonuniform crossover frequencies, where some regions of a chromosome may have higher or lower rates of recombination than others. Thus, while genetic maps provide a useful framework for understanding gene order and distance, they do not directly translate into precise base pair measurements. However, genetic maps can be correlated with physical reality using techniques like genome sequencing.

Answer 95

Each chromosome represents a linkage group, and the genes within that group are subject to recombination events, which can alter their inheritance patterns. The organization of linkage groups helps researchers identify chromosomal locations of genes and understand genetic relationships in multiple-factor crosses, which can further establish linkage groups.

Answer 96

refers to the phenomenon where the occurrence of one crossover event affects the likelihood of another crossover occurring nearby. It can complicate the interpretation of genetic maps.

Answer 97

- A measure used to assess the extent of interference. It is calculated using the formula:

Answer 98

The proportion of single crossover types gives RF between neighbouring gene pairs (e.g., between A and B, and B and C). Accounting for double crossovers refines the distances and reduces the risk of underestimating distances. This method produces a more accurate genetic map as it captures more complex recombination events that simpler two-point tests might miss.

Answer 99

hotspots coldspots

Answer 100

The chi-square (χ²) test is a statistical tool used to evaluate whether there is a significant difference between observed and expected frequencies in categorical data. In genetics, it’s often used to determine if experimental results fit expected ratios (such as Mendelian inheritance ratios). By comparing the observed data with expected outcomes, the chi-square test helps decide whether deviations from expectations are likely due to random chance or other factors, such as gene linkage.

Answer 101

- The null hypothesis (H₀) is a statement used in hypothesis testing that assumes no difference or effect is present in the data. It represents the baseline assumption that any differences between observed and expected values are due to random chance. For example, in a genetic cross, the null hypothesis might state that the observed data follow a 9:3:3:1 ratio, indicating independent assortment. - The goal of a statistical test like chi-square is to either reject or fail to reject the null hypothesis. If the observed data significantly deviates from what is expected, the null hypothesis is rejected, suggesting that some other factor (like gene linkage or environmental influence) may be at play.

Answer 102

1; Define the null hypothesis (H₀): This states that the observed frequencies match the expected frequencies (e.g., "The genes assort independently, and the progeny fit a 9:3:3:1 ratio"). 2. Calculate expected values: Based on the hypothesis, determine how many offspring or results are expected in each category (e.g., in a dihybrid cross with 9:3:3:1 ratio, the expected numbers are based on the total number of offspring). 3. Apply the chi-square formula: χ2=∑(O−E)^2/E Where O is the observed frequency, and E is the expected frequency for each category. 4. Determine the degrees of freedom (df): The degrees of freedom (df) are calculated as: df= (number of categories−1) For example, in a dihybrid cross with four phenotypic categories (9:3:3:1), the degrees of freedom would be df=4−1=3df = 4 - 1 = 3df=4−1=3. 5. Compare the chi-square value with a critical value: Using a chi-square distribution table, locate the critical value based on the degrees of freedom and a chosen significance level, usually p = 0.05. For example, with 3 degrees of freedom, the critical value at p = 0.05 is 7.815. If the calculated chi-square value exceeds this critical value, the null hypothesis is rejected, meaning the differences between observed and expected frequencies are statistically significant and not due to chance.

Answer 103

- If the calculated chi-square value is less than the critical value, the null hypothesis is accepted, indicating the observed variation can be attributed to chance. - If the calculated value exceeds the critical value, the null hypothesis is rejected, implying that the difference between observed and expected values is significant and may indicate phenomena like gene linkage or experimental error.

Answer 104

In the context of fungi, particularly in organisms like Neurospora and other ascomycetes, a tetrad refers to a structure called an ascus that contains four ascospores. This occurs following meiosis, where each ascus typically results from a single meiotic event that produces four genetically distinct spores. The term "tetrad" derives from the fact that four ascospores are formed, reflecting the outcome of one meiotic division followed by mitotic divisions, where the spores are arranged in a linear or clustered fashion.

Answer 105

This refers to an ascus containing only parental type ascospores, meaning all spores have the same allele combinations as the original parents. For instance, in a cross of AABB x aabb, a PD would yield all AABB and aabb spores.

Answer 106

This ascus contains only nonparental (recombinant) type ascospores. This occurs when there is a recombination event between the two loci, resulting in spores with new allele combinations. For example, from the same cross, an NPD would yield only AaBb and aabb spores.

Answer 107

A tetratype ascus contains a mix of parental and recombinant ascospores. For example, it would produce four spores, such as AABB, aabb, AaBb, and Aabb. Tetratypes arise from single crossover events between the loci.

Answer 108

- The ratio of parental ditype (PD) and nonparental ditype (NPD) asci is instrumental in determining linkage between genes. If two genes are unlinked and assort independently, the expected ratio of PD to NPD will be approximately 3:1. - However, if the genes are linked, you would expect a higher frequency of PDs than NPDs. - The formula used is: Linkage = (NDP/Total#ofTetrads) < 1/3 - When the NPD count is significantly lower than one-third of the total tetrads, this indicates that the genes are likely linked, as linked genes produce more parental type asci and fewer recombinant types due to reduced recombination events.

Answer 109

Ordered tetrad

Answer 110

First-division segregation in ascospores happens during Meiosis I when homologous chromosomes carrying different alleles separate without recombination near the gene of interest. This leads to a 2:2 arrangement of alleles (e.g., two ascospores with allele A and two with allele a) within the linear tetrads in the ascus. The lack of crossing-over near the locus ensures that alleles segregate at the first division. It is used to study gene-to-centromere distance, as no recombination indicates the gene is close to the centromere

Answer 111

Second-division segregation in ascospores occurs during Meiosis II when recombination (crossing-over) happens between a gene and its centromere. This recombination causes homologous alleles to remain together through Meiosis I, and only during Meiosis II do the sister chromatids, now carrying different alleles, separate. This results in a 2:2:2:2 or 4:4 arrangement of alleles in the ascus, depending on the pattern of recombination. For example, a gene with alleles A and a may produce a tetrad with alternating ascospores (e.g., A, a, A, a) or two pairs of ascospores (e.g., AA, aa). The key feature of second-division segregation is that recombination between the gene and centromere leads to the separation of alleles later, during the second meiotic division

Answer 112

Involves the examination of ascospores without regard to their arrangement. This is often used when the tetrads cannot be ordered based on the ascospore configuration. In both ordered and unordered analyses, the ratio of PDs, NPDs, and Ts helps infer the distances and linkage between genes, as well as provide insights into the positioning of centromeres relative to the genes being studied.

Answer 113

This occurs during the S phase of the cell cycle when DNA is replicated, allowing for segments of homologous chromosomes to be exchanged. For example, in a heterozygous organism (e.g., Aa), a mitotic crossover can create patches of homozygous cells (AA or aa) adjacent to heterozygous cells (Aa), resulting in observable phenotypic differences, like varying colors in plants or skin patterns in animals. Twin spots signify that genetic recombination has occurred during somatic cell divisions, contributing to genetic diversity within the organism.

Answer 114

Sectored bacterial colonies exhibit distinct areas with different phenotypes, resulting from mitotic recombination within a single colony, while non-sectored colonies show uniform phenotypes throughout. The presence of sectors indicates genetic diversity due to allele segregation, whereas non-sectored colonies reflect genetic consistency. This distinction can help researchers study genetic stability and recombination events in bacterial populations.

Answer 115

Mutations in somatic cells affect only the individual organism in which they occur. Since these cells do not contribute to the next generation, somatic mutations (e.g., in skin or liver cells) are typically not passed on. They can cause forward mutations that may disrupt normal function, potentially leading to cancer or other diseases

Answer 116

- Forward mutations are genetic changes that alter a wild-type (normal) allele into a mutant form, leading to a _new or altered phenotype._ - These mutations are typically the initial step in the process of genetic variation, providing raw material for evolution and are often irreversible without a reverse mutation (reversion).

Answer 117

Mutations in germ-line cells (cells giving rise to sperm or eggs) can be passed to offspring, allowing these mutations to become heritable. Germ-line mutations can also include reverse mutations (reversions), restoring function to a mutated gene, though these are rare.

Answer 118

- Reversions (or reverse mutations) restore a mutant allele back to its original wild-type form, either fully or partially, recovering the gene's original function or phenotype. - Reversions can occur through true reversions (where the original mutation is corrected) or second-site suppressor mutations (where a new mutation elsewhere compensates for the effect of the first mutation).

Answer 119

- A type of substitution mutation where a purine (adenine or guanine) is replaced by another purine, or a pyrimidine (cytosine or thymine) is replaced by another pyrimidine. This maintains the structure of the DNA backbone but can disrupt gene function.

Answer 120

A substitution in which a purine is swapped with a pyrimidine or vice versa. Transversions are more likely to alter the structure of the DNA molecule, potentially causing more dramatic changes to protein function.

Answer 121

This involves the loss of one or more nucleotides from a DNA sequence. Deletions, along with insertions, are often frameshift mutations, shifting the reading frame of the gene and usually disrupting its function.

Answer 122

This type of mutation adds one or more nucleotides into the DNA sequence. Like deletions, insertions can be highly disruptive by shifting the genetic code’s reading frame, often resulting in forward mutations that alter protein function.

Answer 123

- Gene size: Larger genes are more prone to mutations due to having more DNA sequences that can be altered. - DNA sequence context: Certain sequences are more susceptible to mutations, like regions rich in repetitive sequences or specific nucleotides prone to chemical alterations. - Functional constraints: Some gene functions are easy to disrupt but hard to restore. For instance, forward mutations disrupting gene function are often common, while reverse mutations (reversions) that restore function are rare. - Organism complexity: Multicellular organisms generally have higher mutation rates per generation compared to bacteria, partly due to their complex cellular processes. - Germ-line differences: Human sperm show a higher mutation rate than eggs due to continuous cell division in sperm production, which introduces more replication errors over time.

Answer 124

The fluctuation test, developed by Luria and Delbrück, provided evidence that spontaneous mutations arise from random events rather than being directed by environmental pressures. They cultured bacteria in multiple small samples and observed significant variation in mutation rates across colonies, supporting the idea that mutations occur at random, independent of external selective pressures.

Answer 125

Purines: A & G Pyrimidines: C & T

Answer 126

Replica plating, an experiment developed by Joshua and Esther Lederberg, also demonstrated the randomness of mutations. In this technique, bacterial colonies were transferred from one plate to another with a selective agent (e.g., antibiotic). Colonies that survived the antibiotic were shown to already contain mutations before exposure, indicating that spontaneous mutations are pre-existing, rather than induced by the environment. | Left is how it works irl, right is wrong

Answer 127

These findings underscore that mutations occur _spontaneously and randomly_ rather than in response to specific selective pressures. This randomness plays a key role in the evolutionary process by contributing to genetic diversity

Answer 128

- A natural process that can produce mutations by damaging DNA - This process involves the loss of a purine base (adenine or guanine) from the DNA, creating an apurinic site where the base is missing. If unrepaired, this can lead to a mutation during replication.

Answer 129

- A natural process that can produce mutations by damaging DNA - Deamination removes an amino group from cytosine, converting it to uracil. This alteration can cause a mispairing during DNA replication, resulting in mutations.

Answer 130

- A natural process that can produce mutations by damaging DNA - Ultraviolet (UV) light from sunlight can induce thymine dimers, where two adjacent thymine bases become chemically linked. Thymine dimers distort the DNA helix, potentially blocking replication and leading to mutations if not corrected.

Answer 131

- Base tautomerization occurs when DNA bases temporarily shift into alternative chemical structures, called tautomers, which differ slightly in the positioning of hydrogen atoms and bonding patterns. These rare tautomers can alter base pairing properties, leading to unusual pairings during DNA replication. - For example, a tautomeric form of adenine may pair with cytosine instead of its usual partner, thymine. This mispairing introduces an error in the DNA sequence, creating a base mispair that, if not corrected, can propagate through future rounds of replication. - When the DNA is replicated, the presence of these tautomers can result in permanent mutations, as the mispair is incorporated into one of the new DNA strands. This spontaneous change, although rare, is a source of genetic variation and can contribute to mutation rates in cells.

Answer 132

- Slipped mispairing is an error in DNA replication where the DNA strands temporarily misalign, often in regions with repetitive sequences. During this process, the newly synthesised strand may loop out or slip backward, creating extra base pairs or leaving some bases unpaired. - This slippage can lead to insertions or deletions within the DNA sequence, as sections of the strand may be duplicated or skipped altogether. These errors can cause frameshift mutations that alter the reading frame of a gene, potentially disrupting gene function. - Slipped mispairing is especially common in regions with trinucleotide repeats or other short repeat sequences, where it can lead to the expansion or contraction of these repeats, contributing to disorders like Huntington's disease and certain forms of muscular dystrophy when unstable trinucleotide repeats expand too far.

Answer 133

proofreading Base tautomerization Slipped mispairing

Answer 134

A mutagen is any substance or agent that can induce mutations by directly altering DNA. Mutagens can be natural (like UV light) or synthetic (like chemical agents), and they vary in their mechanisms:

Answer 135

Base analogs Intercalators Hydroxylating Alkylating Deaminating

Answer 136

The Ames test is a widely used assay to identify carcinogens—substances that could cause cancer—by testing their ability to induce mutations in bacteria. In this test, bacteria with a specific mutation (often in the gene required for histidine synthesis) are exposed to a suspected mutagen. These bacteria are then grown on petri plates without histidine, where only revertants (cells that have undergone a reverse mutation restoring histidine synthesis) can grow. If the mutagen induces a high rate of reversion mutations, more colonies will appear, suggesting that the substance is likely a carcinogen. The Ames test relies on the correlation between mutagenicity and carcinogenicity, providing a preliminary screening for substances that may damage DNA.

Answer 137

Direct repair Base excision repair Nucleotide excision repair

Answer 138

Some DNA damage can be directly reversed without cutting the DNA backbone. For example, alkyltransferase enzymes can remove alkyl groups from DNA bases, reversing alkylation damage.

Answer 139

Damaged bases are removed and replaced by enzymes like DNA glycosylase, which identifies and removes the damaged base, leaving an AP (apurinic/apyrimidinic) site. An enzyme called AP endonuclease then makes a nick in the DNA backbone at the AP site, allowing DNA polymerase and ligase to fill in the gap.

Answer 140

Nucleotide excision repair (NER): The UvrA, UvrB, and UvrC proteins work together to detect and remove large distortions in the DNA, such as those caused by UV light (e.g., thymine dimers).

Answer 141

This method of repair uses a homologous sequence as a template, resulting in accurate repair without losing genetic information. It occurs during certain cell cycle phases (S and G2) when a sister chromatid is available. HR is precise and is used for genome editing because it maintains sequence fidelity.

Answer 142

This repair mechanism directly joins the broken ends of DNA without needing a homologous template, which can lead to errors, such as small insertions or deletions. It is active throughout the cell cycle, making it a faster but less accurate method than HR.

Answer 143

NHEJ can cause nonhomologous recombination, which may lead to mutations, while HR preserves genetic information.

Answer 144

In methyl-directed mismatch repair (MMR), the cell differentiates between the newly synthesized strand and the template strand based on methylation patterns. Adenine methylase methylates adenine residues in the parent DNA strand, while the new strand remains unmethylated for a short period after replication. The MutS protein detects mismatches, MutL recruits MutH, and MutH cleaves the unmethylated (new) strand at the site of the error. DNA polymerase then synthesizes the correct sequence, and DNA ligase seals the strand. This method allows for accurate correction by identifying the newly made strand as the one to repair.

Answer 145

Cells reserve error-prone repair systems for times when DNA damage is too extensive for high-fidelity mechanisms. For example, sloppy DNA polymerases in the SOS system are capable of bypassing DNA lesions but often introduce mutations. Additionally, microhomology-mediated end-joining (MMEJ), another error-prone mechanism, relies on small regions of sequence similarity and frequently causes deletions or rearrangements. These systems are used as a last resort to allow the cell to survive extreme DNA damage at the cost of increased mutation risk.

Answer 146

- Sloppy DNA polymerases are specialised enzymes used by cells in error-prone repair systems when DNA damage is too severe for high-fidelity repair. In the SOS system, these polymerases can bypass DNA lesions that would normally block replication, allowing the cell to continue dividing despite extensive damage. - However, because sloppy DNA polymerases lack proofreading abilities, they often introduce mutations during repair, making this system a risky but necessary last resort. This approach allows the cell to survive DNA damage that might otherwise be lethal, albeit with an increased chance of accumulating mutations.

Answer 147

- The SOS system is an emergency response in cells activated when DNA damage becomes too extensive for standard repair mechanisms. Triggered by severe DNA damage, the SOS system initiates the production of sloppy DNA polymerases, which are specialised enzymes capable of bypassing damaged DNA regions to allow replication to continue. - These error-prone polymerases help the cell survive by completing DNA synthesis even across lesions that would normally block high-fidelity polymerases. However, their lack of proofreading ability often leads to mutations, as they introduce errors during the repair process. - As a last-resort strategy, the SOS system prioritises cell survival over genomic accuracy, accepting an increased mutation risk. This repair mechanism, although imprecise, enables the cell to tolerate extreme DNA damage that might otherwise result in cell deat

Answer 148

Mutations in genes coding for DNA repair proteins can compromise the cell’s ability to maintain genomic stability, leading to increased rates of mutations and susceptibility to cancer. For example, mutations in DNA repair genes can cause _xeroderma pigmentosum (XP),_ a condition where patients are highly sensitive to UV radiation and prone to skin cancers. DNA repair deficiencies may also contribute to other cancers and diseases involving premature aging, underscoring the importance of functional repair pathways for human health.

Answer 149

Deletions Duplications Inversions Reciprocal translocations

Answer 150

A chromosome segment is reversed in orientation. Inversions can disrupt gene function if they occur within a gene or separate a gene from its regulatory elements.

Answer 151

This type of rearrangement occurs when segments from two non-homologous chromosomes exchange places. While reciprocal translocations do not necessarily result in gene loss, they can disrupt gene function by altering regulatory contexts.

Answer 152

**DNA breakage** refers to the disruption of the DNA double helix, resulting in single-strand breaks (SSBs) or double-strand breaks (DSBs). SSBs involve damage to one strand and are typically repaired efficiently, while DSBs are more severe and require complex repair pathways such as homologous recombination or non-homologous end joining. DNA breakage can arise from endogenous factors like reactive oxygen species or replication stress, as well as exogenous factors such as radiation or chemical agents. If not repaired accurately, DNA breaks can lead to mutations, genomic instability, or chromosomal rearrangements. **Illegitimate crossing over** is an abnormal recombination event occurring between non-homologous or partially homologous DNA sequences. Unlike normal homologous recombination, illegitimate crossing over often results in genetic abnormalities, including deletions, duplications, or translocations. It typically arises due to misalignment during DNA break repair and is associated with mechanisms like non-homologous end joining or microhomology-mediated end joining. This process can contribute to chromosomal abnormalities, cancer, and genetic disorders.

Answer 153

Double-strand breaks in DNA can lead to rearrangements if the repair mechanisms improperly rejoin the fragments. DNA breakage is a primary cause of deletions, duplications, inversions, and reciprocal translocations when fragments are misrepaired or joined incorrectly.

Answer 154

Misalignment during meiosis can result in illegitimate crossing over between non-homologous or misaligned homologous regions, leading to duplications or deletions. This mechanism is particularly relevant in regions with repeated sequences, where incorrect pairing can easily occur.

Answer 155

FISH uses fluorescently labeled DNA probes to bind specific chromosome regions, allowing researchers to detect rearrangement breakpoints. FISH is especially useful for identifying large deletions, duplications, or reciprocal translocations by visually confirming the presence or absence of specific chromosomal regions.

Answer 156

- Rearrangement breakpoints are specific locations on chromosomes where DNA strands have been broken and subsequently rejoined, often in new configurations. These breakpoints are central to chromosomal rearrangements, such as inversions, translocations, deletions, or duplications. - The position of a rearrangement breakpoint can disrupt gene function if it occurs within or near a gene, altering gene expression or creating fusion genes that combine parts of different genes, potentially leading to abnormal cellular behaviour or disease.

Answer 157

homozygotes

Answer 158

heterozygotes haploinsufficiency

Answer 159

- Gene dosage refers to the number of copies of a particular gene present in a cell or organism's genome. It plays a critical role in determining the amount of gene product (such as protein) produced, influencing various biological processes and overall phenotype. - Abnormalities in gene dosage, such as in cases of aneuploidy (where there are extra or missing chromosomes), can lead to dysregulation of gene expression. This can result in developmental disorders or diseases, as certain genes may be overexpressed or underexpressed, disrupting normal cellular functions and homeostasis.

Answer 160

- Deletion loops occur during meiosis when homologous chromosomes align for recombination, and one chromosome contains a deletion—a missing segment of DNA. The loop forms as the normal chromosome attempts to pair with its homologous counterpart, creating a looped-out region where the deleted segment cannot align. - These loops can lead to improper chromosome segregation, resulting in gametes that lack certain genetic information. In heterozygous individuals, where one chromosome has a deletion and the other does not, deletion loops are important for understanding the impact of deletions on gene dosage and potential phenotypic consequences in offspring.

Answer 161

Researchers use deletions as a tool to locate genes by uncovering recessive mutations that would normally be hidden by a dominant allele on the other chromosome. In a typical genetic setup, dominant alleles can mask the effects of recessive mutations if the dominant allele is present on the other chromosome. However, when a deletion removes part of the chromosome, it can eliminate the dominant allele, allowing the recessive mutation to be expressed and its phenotype to become visible. This process is known as uncovering the mutation. By creating deletion heterozygotes—organisms that have one chromosome with a deletion and one with a normal copy—researchers can observe the resulting phenotypes and determine which regions of the chromosome are involved in specific genetic traits. This technique helps in pinpointing the location of genes responsible for various traits.

Answer 162

Tandem duplications Nontandem duplications

Answer 163

Tandem duplications involve the replication of a segment of DNA adjacent to the original sequence, resulting in two or more copies located next to each other on the same chromosome. This type of duplication often occurs in regions with repetitive sequences and can increase gene dosage, potentially leading to enhanced expression of the duplicated genes.

Answer 164

Nontandem duplications occur when a segment of DNA is duplicated and inserted into a different location within the genome, which can be on the same chromosome or a different chromosome. This type of duplication may disrupt normal gene function or regulation by placing the duplicated gene under the control of different regulatory elements, potentially leading to unique phenotypic effects.

Answer 165

unequal crossing-over Bar Bar

Answer 166

Bar mutants are a type of genetic mutation in Drosophila (fruit flies) characterized by a narrow, elongated eye shape due to a deletion in the bar locus on the X chromosome. This mutation leads to a reduced number of photoreceptor cells in the compound eyes, resulting in the distinct bar-shaped appearance.

Answer 167

Double-Bar mutants exhibit an even more pronounced phenotype than Bar mutants, often caused by a homozygous condition or the presence of two closely linked mutations that further enhance the reduction of photoreceptor cells. This results in an even narrower and more elongated eye shape, reflecting a greater degree of gene dosage effect on eye development.

Answer 168

Inversions reorganise the DNA sequence of a chromosome by flipping a segment, but their phenotypic effects depend on whether the inversion’s breakpoints disrupt essential genes or regulatory regions. In pericentric inversions, which include the centromere, or paracentric inversions, which do not, genes interrupted by the breakpoints may lose function, leading to phenotypic consequences. However, if breakpoints do not disrupt genes, the inversion may have no phenotypic effect, though it can still affect fertility and crossover events.

Answer 169

Pericentric inversions are chromosomal rearrangements where a segment of a chromosome is reversed, including the centromere. This inversion changes the order of genes, potentially altering their expression and function. During meiosis, pericentric inversions can complicate homologous chromosome pairing, forming inversion loops. If crossovers occur within these loops, it can produce gametes with duplications and deletions, leading to genetic imbalances that may affect fertility or viability in offspring.

Answer 170

Paracentric inversions are chromosomal rearrangements where a segment of a chromosome is reversed without including the centromere. This inversion alters the order of genes within the inverted region, potentially affecting gene expression and function. During meiosis, paracentric inversions can complicate homologous chromosome pairing, leading to inversion loops. If crossovers occur within these loops, they can produce acentric fragments (without a centromere) and dicentric chromosomes (with two centromeres), causing genetic instability and potentially resulting in nonviable gametes or reduced fertility

Answer 171

Inversion loops form during meiosis when homologous chromosomes with an inversion attempt to align for recombination. The inverted chromosome creates a loop to allow the homologous regions to pair properly, as the sequence order differs due to the inversion. These loops can lead to complications during crossover events, resulting in chromosomal irregularities. If crossovers occur within the loop, it can generate acentric fragments and dicentric chromosomes, which may result in genetic imbalances and affect the viability of gametes.

Answer 172

A reciprocal translocation occurs when segments from two non-homologous chromosomes exchange locations. While this rearrangement does not necessarily cause gene loss, it can have phenotypic consequences if it generates oncogenes or disrupts gene regulation.

Answer 173

diminished pseudolinkage

Answer 174

Inversion heterozygosity can reduce fertility because crossing-over within an inversion loop during meiosis produces abnormal gametes. Paracentric inversions can lead to non-viable offspring due to unbalanced chromosomes, while pericentric inversions can cause acentric fragmentation (loss of chromosome fragments without centromeres) or formation of dicentric chromatids (chromatids with two centromeres), reducing crossover viability. These crossovers are suppressed in inversion heterozygotes, a phenomenon called crossover suppression, and balancer chromosomes are often used in genetic research to maintain certain genotypes without recombination.

Answer 175

In translocation heterozygotes, alternate segregation patterns can produce viable gametes, but adjacent-1 and adjacent-2 segregation patterns often result in semisterility due to unbalanced gametes. Robertsonian translocations (fusion of two acrocentric chromosomes) can also lead to reduced fertility due to uneven chromosome pairing. Translocations may therefore reduce fertility and cause pseudolinkage between genes on different chromosomes. ## Footnote https://www.youtube.com/watch?v=YxxkwCoWcMk

Answer 176

Balancer chromosomes are specially engineered chromosomes used in genetic research to maintain specific mutations in a population while preventing recombination. Their useful features are: 1. **Carry a dominant trait**, so any mutant/balancer heteros will display the dominant phenotype 2. **Contain a lethal allele if homozygous** as in if progeny is balancer/balancer they won't be born and die 3. Progeny showing **recessive allele must be mutant/mutant homozygotes**. By suppressing recombination between the balancer chromosome and the homologous chromosome during meiosis, balancer chromosomes ensure that genetic variants are preserved across generations.

Answer 177

- Adjacent-1 segregation patterns occur during meiosis in individuals with balanced translocations. In this pattern, homologous chromosomes are oriented such that the two chromosomes with translocated segments segregate together, leading to the distribution of one normal and one translocated chromosome to each gamete. - This results in gametes that may carry duplications and deletions of chromosomal segments, which can cause genetic imbalances in the resulting offspring. Adjacent-1 segregation is one of the mechanisms contributing to semisterility in translocation heterozygotes, as many of the gametes produced are nonviable due to these imbalances.

Answer 178

- Adjacent-2 segregation patterns occur during meiosis in individuals with balanced translocations when the homologous chromosomes are oriented such that the translocated segments segregate together. In this pattern, both normal chromosomes from one parent and both translocated chromosomes from the other parent end up in the same gamete. - This results in gametes that contain both normal and duplicated segments, leading to significant genetic imbalances, including duplications and deletions. Adjacent-2 segregation is less common than adjacent-1 segregation but can also contribute to semisterility in translocation heterozygotes, as most gametes produced are typically nonviable due to these imbalances.

Answer 179

- Alternate segregation patterns occur during meiosis in individuals with balanced translocations. In this pattern, homologous chromosomes orient in such a way that one chromosome with a translocation pairs with its corresponding normal chromosome, allowing for the segregation of one normal chromosome and one translocated chromosome into each gamete. - This results in viable gametes that are either normal or contain the balanced translocation, thereby minimizing genetic imbalances. Alternate segregation is the preferred outcome in individuals with balanced translocations, as it leads to a greater chance of producing viable offspring compared to adjacent segregation patterns, which often result in nonviable gametes.

Answer 180

A transposable element (TE), also known as a “jumping gene,” is a DNA sequence that can change its position within the genome through transposition. Transposable elements can move from one genomic location to another, and this movement may disrupt genes, alter gene expression, and contribute to genetic diversity and instability.

Answer 181

These TEs move via an RNA intermediate. They are first transcribed into RNA, which is then reverse-transcribed back into DNA before being inserted into a new genomic location. This class of TEs includes long interspersed elements (LINEs), short interspersed elements (SINEs), and human endogenous retroviruses (HERVs). Retrotransposons do not excise from their original location, resulting in an increase in copy number during transposition.

Answer 182

- Long interspersed elements (LINEs) are a type of transposable element found in genomes, especially in mammals. Typically several thousand base pairs long, they can copy and insert themselves into new genomic locations through retrotransposition. - LINEs are considered autonomous elements as they encode enzymes, such as reverse transcriptase, needed for their mobility. While most LINEs are non-coding, they can influence gene regulation, contribute to genomic diversity, and be associated with diseases when they disrupt normal gene function.

Answer 183

- Short interspersed elements (SINEs) are non-autonomous transposable elements, typically 100 to 400 base pairs long. They do not encode the necessary enzymes for their own mobilization, relying on other elements like LINEs for retrotransposition. - SINEs are prevalent in mammalian genomes, with the Alu element being a notable example. While they generally don't code for proteins, SINEs can affect gene expression and may disrupt gene function when they insert into coding or regulatory regions.

Answer 184

- Human endogenous retroviruses (HERVs) are sequences in the human genome derived from ancient retroviral infections. They are remnants of retroviruses that integrated into the germline of our ancestors, becoming inherited as part of the human DNA. - HERVs make up about 8% of the human genome and can influence gene expression, contribute to genetic diversity, and play a role in immune responses. Some HERVs have been implicated in diseases, including cancer and autoimmune disorders, due to their potential to disrupt normal cellular functions.

Answer 185

DNA transposons move directly as DNA through the action of transposase enzymes, which cut and paste them into new genomic sites. The enzyme transposase catalyses this movement, allowing DNA transposons to excise from one location and insert into another. DNA transposons can be autonomous, meaning they contain a functional transposase gene, or nonautonomous, relying on transposase from other elements for movement.

Answer 186

Half of the human genome consists of transposable elements, primarily LINEs, SINEs, and HERVs. Many TEs in the human genome are defective or nonautonomous elements, meaning they cannot move independently and lack functional genes required for transposition. As a result, they are relatively stable. However, some active autonomous elements still retain their mobility, leading to occasional TE insertions that can disrupt gene function or contribute to genome evolution.

Answer 187

1. Gene mutations 2. Chromosomal rearrangements 3. Gene relocation

Answer 188

Cells employ mechanisms to limit the activity of TEs and maintain genome stability. _piRNAs (PIWI-interacting RNAs)_ play a crucial role in limiting transposable element movement by silencing TE expression, particularly in germ cells. By targeting TE RNA for degradation or preventing translation, piRNAs help suppress TE activity. This regulatory system helps protect the genome from excessive TE mobilisation, which could otherwise lead to harmful mutations and chromosomal instability.

Answer 189

By removing DNA segments, deletions can eliminate gene regulatory regions, leading to loss of gene expression, and sometimes a decrease in fitness if essential genes are affected.

Answer 190

hese create extra gene copies, potentially increasing gene expression in affected regions. Duplications are particularly important for natural selection because they provide redundant gene copies that can acquire mutations over time, generating new gene products with novel functions.

Answer 191

Inversions reorient DNA segments, which can alter regulatory regions and disrupt gene sequences. If an inversion disrupts a gene’s structure or regulation, it may affect fitness, and certain inversions can be adaptive if they increase genetic diversity.

Answer 192

These rearrangements swap segments between chromosomes, potentially placing genes near new regulatory regions and altering their expression. Translocations can bring genes under the influence of telomeric repeats, affecting their stability and expression.

Answer 193

Duplications provide extra gene copies that can undergo independent mutations without affecting the original gene’s function. Over time, these duplicated genes may evolve new or modified functions, forming gene families that contribute to organismal complexity and adaptation. This genetic variation allows natural selection to act on novel traits, promoting evolutionary change. Duplicated genes are particularly valuable for evolution because they can adapt to new roles, increasing fitness in response to environmental pressures.

Answer 194

- Translocations can create incompatibilities in meiosis, leading to reproductive isolation. - Heterozygosity for translocations often causes infertility due to improper alignment of homologous chromosomes, resulting in unbalanced gametes. - Reduced fertility in translocation heterozygotes from different populations limits gene flow, contributing to the development of distinct species over time. - This mechanism is significant in speciation, as chromosomal differences help establish new populations that can no longer interbreed.

Answer 195

Aneuploidy refers to a condition in which an organism has an abnormal number of chromosomes, resulting in an unbalanced genome. This can involve either a surplus or shortage of chromosomes relative to the normal set.

Answer 196

Monosomy is a type of aneuploidy where an individual has only one copy of a particular chromosome instead of the normal two, which can severely impact development.

Answer 197

Trisomy is another type of aneuploidy where an individual has three copies of a chromosome instead of two. Trisomies can cause developmental disorders due to gene dosage imbalance.

Answer 198

- Autosomal aneuploidy is generally more harmful than sex chromosome aneuploidy because it disrupts gene dosage across numerous essential developmental genes, often leading to abnormalities or lethality. - Each autosome contains many genes, so any deviation from the normal two copies can have extensive effects, while sex chromosome aneuploidy is more tolerable due to mechanisms like X-chromosome inactivation in females, which balances gene dosage. - Aneuploid organisms often arise from meiotic nondisjunction, where chromosomes fail to separate properly during meiosis, resulting in unbalanced gametes. - When these gametes fertilise, they produce an aneuploid zygote with an abnormal chromosome number.

Answer 199

- Mosaic organisms have two or more genetically distinct cell lines within the same individual. - This condition can arise from rare mitotic nondisjunction or chromosome loss during early embryonic cell divisions. - If mitotic nondisjunction or chromosome loss occurs in a developing embryo, some cells may have a normal chromosome number while others have an abnormal count, leading to mosaicism. - Gynandromorphs are an example of mosaic organisms that display both male and female characteristics due to mosaicism in sex chromosomes

Answer 200

N euploids aneuploids

Answer 201

Autopolyploids have multiple sets of chromosomes originating from the same species, leading to polyploidy through duplication of a single genome. Autopolyploids form bivalents during meiosis and can produce viable gametes if their chromosome sets are even.

Answer 202

- Bivalents are structures formed during meiosis when homologous chromosomes pair up and align along their lengths, resulting in a connection between the homologues. Each bivalent consists of two homologous chromosomes, each containing two sister chromatids. - This pairing allows for crossing over and genetic recombination to occur, which increases genetic diversity in the resulting gametes. Bivalents are typically observed during prophase I of meiosis.

Answer 203

Allopolyploids, also known as amphiploids, are hybrids that possess complete chromosome sets from two different species. They achieve stable polyploidy as each chromosome set has a corresponding homologous partner, allowing them to function as polyploids and form fertile hybrids.

Answer 204

Organisms with an odd number of chromosome sets, like triploids (3x), often experience sterility due to their inability to form balanced pairs during meiosis. This improper pairing leads to uneven chromosome segregation and unbalanced gametes, resulting in infertility. In contrast, polyploids with an even number of chromosome sets are usually fertile because each chromosome has a homologous partner, allowing for proper pairing.

Answer 205

Monoploid plants, which contain only one set of chromosomes, allow breeders to observe and select favourable mutations without genetic masking from homologous chromosomes. This is useful for producing homozygous plants more quickly through parthenogenesis, the development of embryos from unfertilised gametes.

Answer 206

- Parthenogenesis is a form of asexual reproduction in which an egg develops into an organism without fertilisation by a sperm. This process can occur in certain plants, invertebrates, and some vertebrates, such as reptiles and amphibians. - In parthenogenesis, the offspring are typically clones of the mother, although mechanisms like genetic recombination may occur in some species, allowing for variation among the offspring.

Answer 207

Polyploid plants, especially tetraploids and other polyploids with an even chromosome count, can be used to develop new species with advantageous traits, such as increased size or resilience. Plant breeders often induce polyploidy in crops to improve productivity or resistance, taking advantage of the genetic diversity and robustness that polyploidy provides.

Answer 208

Polyploidization

Answer 209

subfunctionalization neofunctionalization

Answer 210

- Scientists have detected evidence of ancient whole-genome duplications through genomic comparisons, finding extra homologous regions or blocks of duplicated genes in various species. These regions indicate that entire genomes were copied in the past, contributing to current genetic diversity. - In species like O. sativa, researchers have identified multiple sets of homologous genes that suggest ancient polyploidization events, confirming that whole-genome duplications are a key evolutionary mechanism influencing the genetic complexity of many plants and animals today.

Answer 211

Alkyltransferases Photolyase

Answer 212

- Muller’s lethal tagging test is a genetic technique used to locate mutations that cause lethality. By inducing random mutations in an organism's genome, researchers can tag specific gene locations with lethal mutations, effectively “marking” those regions. - This method allows for the identification of genes essential for survival, as only non-lethal mutations will pass through generations. Muller’s lethal tagging is particularly useful in mapping essential genes and studying genetic linkage.

Answer 213

Directionality: - DNAP reads the template strand 3' to 5' and synthesises the new strand 5' to 3' by adding nucleotides to the 3'-OH end. Attachment Requirement: - DNAP requires a free 3'-OH group to attach nucleotide bases, so it cannot start synthesis alone Deoxynucleotide triphosphates (dNTPs): - These include dATP, dCTP, dGTP, and dTTP, which provide both the building blocks and the energy required for synthesis. DNAP catalyses the incorporation of each dNTP by forming phosphodiester bonds between the 3'-OH of the growing strand and the incoming dNTP. Primer: - Typically a short RNA segment, the primer provides the necessary 3'-OH group that initiates DNA synthesis. DNAP extends the primer by adding nucleotides, forming the new DNA strand.

Answer 214

Design the Primer: Create a short DNA sequence (primer) that is complementary to a specific region on the template strand where you want to start synthesis. Mix Primer with DNA: Combine your designed primer with the DNA sample in a reaction tube. The primer will help guide DNA polymerase to the correct starting point. Denature the DNA: Heat the mixture to a high temperature, causing the double-stranded DNA to separate into single strands by breaking the hydrogen bonds between them. Anneal the Primer: Lower the temperature so that the primer can bind (anneal) to its complementary sequence on the template strand. Extend the Primer: Add DNA polymerase to the reaction. Starting from the primer, DNA polymerase will add nucleotides, extending the primer and creating a new complementary DNA strand.

Answer 215

1. Preparation: A single-stranded DNA template, DNA polymerase, normal deoxynucleotide triphosphates (dNTPs), and specially modified dideoxynucleotide triphosphates (ddNTPs) are needed. The ddNTPs lack a 3'-OH group, so once incorporated, they prevent further strand extension. 2. Reaction Mixture: The DNA template is mixed with a primer that binds to a known starting point, along with DNA polymerase, dNTPs, and a small amount of each ddNTP (ddATP, ddTTP, ddCTP, ddGTP) labelled with fluorescent dyes for detection. 3. Chain Termination: DNA polymerase begins to synthesise a complementary strand, incorporating dNTPs as usual. When a ddNTP is randomly incorporated, the strand elongation stops. This creates DNA fragments of varying lengths, each ending at a ddNTP. 4. Separation and Detection: The fragments are then separated by capillary electrophoresis, which sorts them by length. Each ddNTP has a unique fluorescent label, allowing the sequence of bases to be detected as they pass through a laser. 5. Reading the Sequence: The order of fluorescent signals corresponds to the DNA sequence, with each colour indicating a specific base. The sequence of the original DNA template is then reconstructed.

Answer 216

1. Benefits of Sanger Sequencing: - High Fidelity: Sanger sequencing has very high accuracy, making it reliable for precise sequencing tasks. - Long Read Length: It can read up to 900 base pairs (bp) per sequence, which is beneficial for sequencing longer DNA regions without errors. - Cost-Effective for Smaller Projects: For targeted sequencing or smaller-scale projects, Sanger sequencing is affordable and practical. 2. Issues with Sanger Sequencing: - Inefficient for Large Genomes: Sequencing large genomes with Sanger is slow and costly. For example, the Human Genome Project used Sanger sequencing to map the first human genome at a cost of $3 billion in 2001. - Requires Mapping and Gap-Filling: Large genome projects need initial mapping and scaffolding to organise sequence fragments, followed by additional sequencing to fill gaps, adding time and complexity to the process.

Answer 217

- DNA Fragmentation: The genome is randomly broken into thousands or millions of smaller DNA fragments using either mechanical methods, like sonication, or enzymatic digestion. This fragmentation ensures that different parts of the genome are sequenced multiple times, creating overlapping regions that are essential for accurate reconstruction. - Sequencing Fragments: Each DNA fragment is sequenced individually using high-throughput sequencing techniques. The overlap between fragments means that each section of the genome is covered by multiple fragments, which increases accuracy and helps detect errors or inconsistencies in the sequencing process. - Assembly of Overlapping Sequences: Specialised bioinformatics software is used to align and match the overlapping ends of the fragments. By finding these overlaps, the software assembles the short sequences back into a continuous sequence, reconstructing the original genome accurately.

Answer 218

- DNA Preparation and Fragmentation: The DNA sample is fragmented into short pieces, typically around 200–300 base pairs. Adaptors, which are short, synthetic DNA sequences, are then attached to each fragment’s ends to allow attachment to the sequencing flow cell. - Flow Cell Attachment and Amplification: The flow cell is a specialised glass surface with millions of sites for DNA attachment. The DNA fragments bind to complementary sequences on the flow cell and are then amplified in place to form clusters of identical DNA molecules, ensuring sufficient signal strength for detection. - Sequencing by Synthesis: Fluorescently labelled nucleotides are introduced along with DNA polymerase. As the polymerase adds each complementary nucleotide to a growing DNA strand, the fluorescent label is detected by a camera. Each of the four nucleotides (A, T, C, G) has a distinct fluorescent colour, allowing for precise detection. - Data Collection and Base Calling: The flow cell is imaged after each nucleotide incorporation, capturing the fluorescence from each cluster. A computer records and processes these images to determine the sequence of nucleotides for each fragment. - Assembly and Analysis: The short sequences are aligned and assembled using bioinformatics tools to reconstruct the original DNA sequence. This step involves piecing together overlapping sequences from different fragments to generate a complete and accurate sequence

Answer 219

* Single-molecule real-time (SMRT) sequencing (PacBio) * Nanopore Sequencing (Oxford Nanopore Technologies)

Answer 220

- DNA Preparation: The DNA is fragmented and circularised by adding adaptors, creating a SMRTbell template. This closed-loop structure allows DNA polymerase to continuously read the template in multiple passes, which improves sequencing accuracy. - Zero-Mode Waveguides (ZMWs): The sequencing takes place in tiny wells called zero-mode waveguides (ZMWs), each containing a single DNA molecule bound to a DNA polymerase. ZMWs are engineered to detect fluorescent signals from individual molecules, enabling single-molecule sequencing. - Sequencing by Synthesis: Fluorescently labelled nucleotides are introduced into the ZMWs. As DNA polymerase incorporates each nucleotide into the growing strand, a unique fluorescent signal is emitted and captured by a detector in real time. Each base (A, T, C, G) has a different coloured label, allowing accurate identification. - Real-Time Data Collection: The signal from each incorporated nucleotide is recorded continuously, allowing the instrument to capture the sequence in real time. The duration of each fluorescent signal also provides additional information about the chemical environment and can help distinguish between different DNA modifications. - Assembly and Analysis: The sequencing data from individual molecules is processed to generate long reads, often tens of thousands of base pairs. Multiple passes over the circular SMRTbell template allow for error correction, resulting in highly accurate long-read data.

Answer 221

- DNA Preparation and Loading: Long DNA molecules are prepared for sequencing, with each strand passed through an adaptor complex that guides it into the nanopore. Unlike other methods, nanopore sequencing can handle long DNA fragments, often tens of thousands of base pairs, which allows for sequencing entire genes or large genomic regions without fragmentation. - Nanopore Detection: The DNA molecule is threaded through a biological nanopore embedded in a membrane, with an electric current applied across the membrane. As the DNA bases (A, T, C, G) pass through the nanopore, each base disrupts the electrical current in a unique way. - Signal Detection and Base Calling: Changes in current are recorded as each base moves through the pore, generating a unique signal profile. Software then translates these electrical signals into the corresponding DNA sequence, identifying bases in real time as the DNA passes through. - Data Processing and Analysis: The raw signal data is processed into DNA sequences using advanced base-calling algorithms. Nanopore sequencing allows for direct sequencing of native DNA strands, which can include modified bases, providing insights into epigenetic modifications without extra processing.

Answer 222

- Sequencing and Read Generation: The DNA sample is sequenced, resulting in many short reads. These reads are fragmented pieces of DNA that collectively cover the entire genome of the sample organism. - Alignment to Reference Genome: Each read is aligned to the reference genome—a known, closely related genome—using specialised software. The software places each read at the location on the reference genome where it best matches, based on sequence similarity. - Consensus Building: Once aligned, the reads are combined to create a consensus sequence. This consensus represents the sample genome while incorporating any differences, such as mutations or structural variants, between the sample and the reference genome. - Variant Identification: Differences between the sample and reference genome, such as single nucleotide polymorphisms (SNPs), insertions, or deletions, are identified. This allows researchers to study genetic variations in the sample genome relative to the reference.

Answer 223

- Sequencing and Read Collection: The DNA of the organism is fragmented and sequenced, producing a large set of short reads. These reads are overlapping fragments that cover different parts of the genome, and they serve as the raw data for assembly. - Overlap Detection and Contig Formation: Specialised software analyses the reads to find overlapping sequences, which indicate where fragments originate from adjacent regions of the genome. These overlaps are used to join reads together into contigs (continuous sequences). Contigs represent longer stretches of the genome, but may still contain gaps. - Scaffolding: Contigs are further organised into scaffolds using paired-end reads (reads that come from opposite ends of the same DNA fragment) or long reads. These scaffolds approximate the order and orientation of contigs, effectively bridging gaps between them to form longer, more complete segments of the genome. - Gap Filling and Error Correction: Additional techniques, such as sequencing at higher coverage or using long-read sequencing, help close gaps between scaffolds. Error correction software is applied to refine the assembly by correcting any sequencing errors or resolving ambiguous regions.

Answer 224

- Eusocial insects (bees) - Plants, fungi, and algae can switch between haploid and diploid stage (alternation of generations)

Answer 225

- Polyploidy is common in plants, invertebrates, reptiles and amphibians – Polyploidy is extremely rare in mammals

Answer 226

Autopolyploid Allopolyploid Amphidiploid

Answer 227

- Hybridisation of Wheat and Rye: Triticale is initially produced by crossbreeding wheat and rye. Wheat, a hexaploid (six sets of chromosomes, 2n = 6x = 42), is crossed with rye, a diploid (two sets of chromosomes, 2n = 2x = 14). The resulting offspring have three sets of chromosomes from wheat and one set from rye, leading to partial sterility due to mismatched chromosomes. - Chromosome Doubling: To overcome sterility, the chromosomes are doubled using colchicine, a chemical that disrupts normal cell division and allows chromosome duplication. This treatment produces a fertile octoploid (2n = 8x = 56) with four sets of chromosomes from wheat and two from rye, allowing successful chromosome pairing during meiosis and restoring fertility.

Answer 228

- Up to 4 million loci can be genotyped simultaneously for approximately $100 - Companies like 23andMe and Ancestry use microarrays to access SNPs in your DNA to infer ethnogeographic background and predictors of health

Answer 229

In genetics, admixture refers to the mixing of distinct populations that results in the combination of genetic material from different ancestral groups. This process occurs when individuals from previously separated populations interbreed, creating offspring with a genetic blend of both lineages. Admixture can reveal historical migration patterns and gene flow between populations, influencing genetic diversity and sometimes introducing beneficial traits. It is often studied using genetic markers to identify contributions from each ancestral group within the mixed population.

Answer 230

Restriction enzyme cloning is a technique used to insert a DNA fragment into a vector by cutting both with the same restriction enzymes. These enzymes recognise specific DNA sequences and create cuts, often leaving sticky ends (overhangs) or blunt ends that facilitate the joining of the DNA fragment and vector. The cut fragment and vector are then ligated (joined) with DNA ligase, creating a recombinant DNA molecule that can be introduced into host cells for replication or expression. This method is widely used for gene cloning, enabling researchers to insert specific genes into vectors for study or practical applications.

Answer 231

Gateway cloning is a recombination-based method that allows for the efficient transfer of DNA fragments between vectors without the need for restriction enzymes. It uses two main steps, BP and LR reactions, to move the gene of interest through the cloning system. BP Reaction: The gene of interest, flanked by specific recombination sites (attB), is mixed with a donor vector containing compatible attP sites. Through recombination catalysed by BP Clonase, an entry clone is created with attL sites flanking the gene, allowing it to be used in downstream applications. LR Reaction: The entry clone (with attL sites) is mixed with a destination vector containing attR sites. LR Clonase catalyses recombination between attL and attR sites, transferring the gene of interest into the destination vector, now ready for expression in a desired system.

Answer 232

TOPO cloning is a rapid method of cloning DNA that uses a topoisomerase enzyme to insert a DNA fragment into a vector without the need for restriction enzymes or ligase. In this method, the vector is pre-linearised and modified with a topoisomerase enzyme, which remains covalently attached to the vector ends, allowing the DNA fragment to be efficiently joined. Typically, PCR-amplified DNA with single 3’-A overhangs pairs with complementary T-overhangs on the TOPO vector, enabling quick and directional insertion. This technique is commonly used for cloning PCR products directly, as it requires fewer steps and is time-efficient.

Answer 233

Gibson Assembly is a cloning method that allows for the seamless joining of multiple DNA fragments in a single reaction without the need for restriction enzymes. In this method, DNA fragments with overlapping ends are combined with a mix of enzymes that includes an exonuclease to chew back the ends, a DNA polymerase to fill in gaps, and DNA ligase to seal the nicks. The exonuclease creates complementary overhangs on adjacent fragments, allowing them to anneal and be joined into a continuous sequence. Gibson Assembly is widely used for constructing large DNA molecules or assembling complex gene constructs with high efficiency.

Answer 234

Golden Gate: In this method, Type IIS enzymes (e.g., BsaI or BsmBI) are used to generate sticky ends on both the vector and insert DNA fragments. The fragments are then ligated in a single reaction, with the overhangs ensuring correct orientation and positioning. This process allows for the efficient assembly of multiple DNA parts in a one-pot reaction. MoClo: The Modular Cloning (MoClo) system builds on Golden Gate by using a set of standardised vectors and plasmids for constructing complex genetic circuits. It simplifies the assembly of multi-part constructs, enabling high-throughput cloning of large DNA assemblies with minimal error.

Answer 235

Transgenic organisms are organisms that have been genetically modified to carry a gene from a different species. This is typically achieved through the introduction of foreign DNA into their genome using techniques such as gene cloning or genetic engineering. The inserted gene, known as a transgene, can confer new traits or characteristics, such as disease resistance in plants or the ability to produce a specific protein in animals.

Answer 236

Transgenic cells and organisms are used as protein factories to produce pharmaceutical proteins that are difficult to obtain through conventional methods. While some human proteins, like insulin, can be produced in bacteria, others require post-translational modifications that bacteria cannot perform. Transgenic mammalian cells in liquid culture are commonly used for producing complex proteins, such as Factor VIII, which is critical for blood clotting, and erythropoietin (EPO), which stimulates red blood cell production. These cells can carry the gene for the protein, allowing large-scale production for therapeutic use. In some cases, plant cells are also used to produce specific proteins, such as glucocerebrosidase, which is used to treat Gaucher’s disease, a rare genetic disorder. These systems offer a cost-effective alternative for producing therapeutic proteins that require complex modifications not possible in bacteria.

Answer 237

1. DNA Accessibility: The DNA must be in an accessible state. In eukaryotes, DNA is packaged into chromatin, which can be tightly (inactive) or loosely (active) packed. Modifications to histones and chromatin remodeling proteins help make certain regions of DNA more accessible to RNA polymerase for transcription. 2. Transcription Factors: These proteins guide RNA polymerase to specific DNA regions called promoters. They can be activators, which enhance transcription, or repressors, which inhibit it. Transcription factors help recruit RNA polymerase to the correct spot on the DNA and ensure transcription begins at the appropriate time and place

Answer 238

DNA being sufficiently unwound. This means the reduction of its methylation (which makes it hard for RNA Pol-II to bind and pull it apart, binds to CG). Also can be helped by acetylation of histone making DNA accessible

Answer 239

1. Chromatin Remodeling: They recruit complexes that alter the position or removal of nucleosomes, opening up the DNA. 2. Histone Modifications: They recruit enzymes that add or remove chemical marks (like acetyl or methyl groups) to histones, making the DNA more or less accessible. 3. DNA Looping: They can bring distant regulatory regions (enhancers) closer to the gene's promoter, facilitating transcription. 4.Coactivators/Corepressors: They interact with other proteins that either open (coactivators) or close (corepressors) the chromatin structure.

Answer 240

Pubertal timing is a complex genetic trait influenced by many factors. It involves multiple genes that regulate hormone signaling, growth, and metabolic processes, with small effects from each gene. Environmental factors like nutrition, stress, and exposure to chemicals also play a significant role in determining when puberty occurs. The trait shows high heritability, with family studies suggesting genetic influence, but gene-environment interactions are crucial. Sex differences exist, as females tend to enter puberty earlier than males. While GWAS have identified some genetic variants related to pubertal timing, the trait remains multifactorial and influenced by both genetic and environmental factors.

Answer 241

Peto's paradox refers to the observation that cancer incidence does not increase proportionally with an organism's size or lifespan, despite larger organisms having vastly more cells and longer periods for potential mutations to accumulate. For example, whales and elephants, which have far more cells than humans, do not exhibit higher cancer rates. The paradox highlights how larger and longer-lived species must have evolved highly efficient cancer suppression mechanisms. These mechanisms may include enhanced DNA repair, apoptosis, tumour suppressor gene activity, or immune surveillance. For instance, elephants have multiple copies of the TP53 gene, which plays a critical role in tumour suppression, whereas humans have only a single pair. Peto’s paradox underscores the complexity of cancer biology and the evolutionary adaptations that mitigate its risks.

Genetics Final Flashcards

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