Chapter 17

Question 1

Discuss Mendel’s experiments on pea plants and how they contribute to our knowledge of inheritance

Answer 1

Gregor Mendel, often called the “father of genetics,” was an Austrian monk who made significant contributions to the field of biology through his experiments on pea plants. His work laid the foundation for our understanding of inheritance

Mendel chose pea plants for his experiments due to their rapid life cycle and the production of numerous seeds. He conducted his research over a decade, experimenting with almost 30,000 pea plants

Through his experiments, Mendel developed three principles of inheritance that described the transmission of genetic traits:
1. Principle of Segregation: Each inherited trait is defined by a pair of genes. Offspring receive one genetic allele from each parent

2. Principle of Independent Assortment: Genes for different traits are sorted separately so that the inheritance of one trait is not dependent on the inheritance of another
3. Principle of Dominance: An organism with at least one dominant allele will display the effect of the dominant allele

Interestingly, Mendel’s experiments showed that traits are passed on from one generation to the next in a predictable manner. He demonstrated that traits are inherited independently of each other and that they can be dominant or recessive. His experiments also proved that genes are discrete and do not blend together

These principles greatly expanded our understanding of genetic inheritance and led to the development of new experimental methods. Today, they form the basis of Mendelian genetics, a subfield of genetics that focuses on the study of inheritance and genetic variation. Mendel’s work applies to all living things that reproduce sexually, including humans

Question 2

Define allele

Answer 2

An allele is one of two or more versions of a gene that an individual inherits from each parent. These versions can be a single base or a segment of bases at a given genomic location. If the two alleles are the same, the individual is homozygous for that allele. If the alleles are different, the individual is heterozygous. Typically, we call them either normal or wild-type alleles, or abnormal, or mutant alleles

Question 3

Describe the difference between dominant and recessive traits

Answer 3

Dominant Traits: These traits are always expressed when the connected allele is dominant, even if only one copy exists. Dominant traits mask the expression of recessive alleles. For example, the allele for brown eyes is dominant, so an individual with at least one brown eye allele will have brown eyes

Recessive Traits: These traits are expressed only if both the connected alleles are recessive. If one of the alleles is dominant, then the associated characteristic is less likely to manifest. For instance, the allele for blue eyes is recessive, so an individual must have two blue eye alleles to have blue eyes

In summary, dominant traits can be inherited from just one parent and are expressed even if only one copy of the dominant allele is present. On the other hand, recessive traits require two copies of the recessive allele to be expressed and must be inherited from both parents

Question 4

Distinguish between genotype and phenotype

Answer 4

Genotype: This refers to the genetic makeup of an individual, the specific combination of genes they inherit from their parents. It’s like the blueprint that determines various traits and characteristics. For each individual trait, a cell contains instructions on two alleles, which are alternative forms of the gene obtained from the mother and the father. An individual’s genotype refers to the combination of these two alleles, and can be either homozygous (the alleles are the same) or heterozygous (the alleles are different)

**Phenotype:* This is the observable physical, physiological, and behavioral traits that result from the interaction between an individual’s genes and their environment. It’s what you can see and measure, such as hair color, height, and the presence of certain diseases. The sum of an organism’s observable characteristics is their phenotype. A key difference between phenotype and genotype is that, while genotype is inherited from an organism’s parents, the phenotype is not

In simple terms, genotype is the genetic code, while phenotype is how those genes manifest in the actual person

Question 5

Predict the outcome of genetic crosses using a Punnett square

Answer 5

1. Identify the genotypes of the parents: The first step is to identify the genotypes (the combination of genes) of the parent organisms
2. List all possible combinations of alleles: All possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid. This represents their meiotic segregation into haploid gametes
3. Fill in the Punnett square: Each box in the Punnett square represents a possible combination of alleles from the parents. Fill in each box with the combination of alleles that corresponds to that row and column
4. Analyze the results: The completed Punnett square gives the probable outcome of the genetic cross. You can determine the genotypic ratio by counting the number of occurrences of each genotype. Based on the possible genotypes, you can assess the phenotypes

Question 6

Define Mendel’s law of segregation and law of independent assortment

Answer 6

Law of Segregation: This law states that the two alleles for each trait segregate, or separate, during the formation of gametes. During the formation of new zygotes, the alleles will combine at random with other alleles. The law of segregation ensures that a parent, with two copies of each gene, can pass on either allele. Both alleles will have the same chance of ending up in a zygote

Law of Independent Assortment: This law states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. This law describes the random inheritance of genes from maternal and paternal sources

Question 7

Relate the behavior of chromosomes during meiosis to Mendel’s laws of inheritance

Answer 7

Mendel’s Law of Segregation and Meiosis: This law states that the two alleles for each trait segregate, or separate, during the formation of gametes. This is mirrored in the process of meiosis, where homologous chromosomes separate so that each sperm or egg receives just one member of the pair. This process is known as segregation

Mendel’s Law of Independent Assortment and Meiosis: This law states that the alleles of two (or more) different genes get sorted into gametes independently of one another. During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs. The sorting of chromosomes from each homologous pair into pre-gametes appears to be random. This random assortment of genes is the physical basis for Mendel’s law of independent assortment

In summary, the behavior of chromosomes during meiosis can explain why genes are inherited according to Mendel’s laws. The physical movement of chromosomes during meiosis corresponds to the segregation and independent assortment of genes, providing a physical basis for Mendel’s laws of inheritance

Question 8

Explain the concept of probability and calculate probabilities of inheritance

Answer 8

In genetics, probability is used to predict the chances of an offspring inheriting a particular trait. For instance, if the event you were looking for was a wrinkled pea seed, and you saw it 1,850 times out of the 7,324 total seeds you examined, the empirical probability of getting a wrinkled seed would be 1,850 / 7,324 = 0.253, or very close to 1 in 4 seeds

Now, let’s move on to calculating probabilities of inheritance:
In genetics, the calculation of inheritance probabilities often involves the use of Punnett squares. However, for complex crosses involving many genes, it becomes more practical to use probability calculations
Two key rules:
The Product Rule: The joint probability of two independent events (both occurring) is the product of their individual probabilities. This rule is used when we are interested in the probability of two events happening at the same time.

The Sum Rule: The combined probability of two mutually exclusive events (either occurring) is the sum of their individual probabilities. This rule is used when we are interested in the probability of either of two events happening

For example, consider a genetic cross between two heterozygous pea plants (Rr x Rr), where ‘R’ represents the dominant allele for round seeds and ‘r’ represents the recessive allele for wrinkled seeds. Using the product rule, the probability of an offspring inheriting two ‘r’ alleles (and thus having wrinkled seeds) is 1/2 (chance of inheriting ‘r’ from the first parent) * 1/2 (chance of inheriting ‘r’ from the second parent) = 1/4

Question 9

Apply the product rule to problems involving genetic crosses

Answer 9

1. Identify the individual probabilities: Determine the probability of each individual event. In a genetic cross, this would be the probability of inheriting a particular allele from each parent
2. Multiply the probabilities: Multiply the individual probabilities together to get the joint probability. This will give you the probability of both events (i.e., inheriting both alleles) occurring together

For example, consider a cross between two heterozygous pea plants (Rr x Rr), where ‘R’ represents the dominant allele for round seeds and ‘r’ represents the recessive allele for wrinkled seeds. The probability of an offspring inheriting an ‘r’ allele from one parent is 1/2, and the same is true for the other parent. Using the product rule, the probability of an offspring inheriting two ‘r’ alleles (and thus having wrinkled seeds) is 1/2 (chance of inheriting ‘r’ from the first parent) * 1/2 (chance of inheriting ‘r’ from the second parent) = 1/4

Remember, the product rule assumes that the events are independent, meaning the outcome of one event does not affect the outcome of the other. In genetics, this is generally a safe assumption due to the law of independent assortment

Question 10

Discuss sex Chromosomes and describe X-Linked inheritance patterns

Answer 10

Sex Chromosomes: Sex chromosomes are a type of chromosome involved in sex determination. Humans and most other mammals have two sex chromosomes, X and Y, that in combination determine the sex of an individual. Females have two X chromosomes in their cells, while males have one X and one Y. The X chromosome is always present as the 23rd chromosome in the ovum, while either an X or Y chromosome may be present in an individual sperm

X-Linked Inheritance Patterns: X-linked inheritance refers to the patterns of inheritance of genes located on the X chromosome. There are two main types of X-linked inheritance: X-linked recessive and X-linked dominant

• X-linked recessive: X-linked recessive disorders are caused by variants in genes on the X chromosome. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a variant would have to occur in both copies of the gene to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females
• X-linked dominant: X-linked dominant disorders are caused by variants in genes on the X chromosome. In males (who have only one X chromosome), a variant in the only copy of the gene in each cell causes the disorder. In females (who have two X chromosomes), a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms of the disorder than males

A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons. This is because males inherit their X chromosome from their mother and their Y chromosome from their father

Question 11

Explain why X-linked recessive traits are more likely to occur in males

Answer 11

X-linked recessive traits are more likely to occur in males due to the structure of their sex chromosomes. Males have one X and one Y chromosome. If a male inherits a recessive allele (or a “bad” allele) on his X chromosome from his mother, he doesn’t have a second X chromosome (like females do) to potentially carry the dominant allele (or a “good” allele) to mask the effects of the recessive one

In contrast, females have two X chromosomes. So, even if they inherit a recessive allele on one X chromosome, they could still have a dominant allele on the other X chromosome that can mask the effects of the recessive allele. This is why females are typically carriers of X-linked recessive traits, while males are more likely to express these traits

Question 12

Relate dominant and recessive traits to protein function

Answer 12

• Dominant Traits: A dominant trait is expressed when at least one dominant allele is present. The dominant allele codes for a functional protein that results in the expression of the trait. Even if there’s a recessive allele present, the functional protein from the dominant allele can often carry out the necessary function, masking the effect of the recessive allele
• Recessive Traits: A recessive trait is expressed when two copies of a recessive allele are present. Recessive alleles often code for a non-functional or less functional protein. If a dominant allele is present, the functional protein it codes for can often compensate for the lack of function of the protein coded by the recessive allele

In the case of X-linked recessive traits, males are more likely to express these traits because they have only one X chromosome. If the X chromosome carries a recessive allele (which codes for a non-functional or less functional protein), there’s no corresponding allele on the Y chromosome to mask its effect

Question 13

Discuss other forms of inheritance (pleiotropy, incomplete dominance and codominance)

Answer 13

1. Pleiotropy: Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. For example, a human genetic disorder called Marfan syndrome is caused by a mutation in one gene, yet it affects many aspects of growth and development, including height, vision, and heart function. This is an example of pleiotropy, or one gene affecting multiple characteristics
2. Incomplete Dominance: Incomplete dominance is when a dominant allele, or form of a gene, does not completely mask the effects of a recessive allele, and the organism’s resulting physical appearance shows a blending of both alleles. For example, in the snapdragon, Antirrhinum majus, a cross between a homozygous white-flowered plant and a homozygous red-flowered plant will produce offspring with pink flowers. This type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance
3. Codominance: Codominance refers to a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual. That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or animal that has more than one pigment color. For example, people with the AB blood type have one A allele and one B allele. Because both alleles are expressed at the same time, their blood type is AB

Question 14

Discuss how the environment plays a critical role in determining the expression of traits

Answer 14

1. Internal and External Factors: Both internal and external environmental factors can influence gene expression. Internal factors include aspects like hormones and metabolism, while external factors can include drugs, chemicals, temperature, and light
2. Influence on Phenotype: Environmental factors such as diet, temperature, oxygen levels, humidity, light cycles, and the presence of mutagens can all impact which of an organism’s genes are expressed, which ultimately affects the organism’s phenotype. Even genetically identical organisms exposed to controlled experimental conditions can have different phenotypes, pointing to the power of subtle environmental differences on gene expression
3. Sex-Influenced and Sex-Limited Traits: The environment can also influence the expression of sex-influenced and sex-limited traits. For example, male-pattern baldness is a sex-influenced trait that can be expressed in women under high-stress situations, which can lead to increased production of certain hormones
4. Drugs and Chemicals: The presence of drugs or chemicals in an organism’s environment can also influence gene expression. For instance, certain chemicals can affect development, as seen in the case of cyclops fish
5. Environmental Traits: Where a person lives can shape and form who they become because their environment is the main determinant of their personalities and individual characteristics

Question 15

Explain why polygenic traits usually show a continuum of phenotype variation

Answer 15

Polygenic traits are controlled by multiple genes, each of which contributes a small effect to the overall phenotype. Because of this, polygenic traits often show a continuum of phenotypic variation, rather than discrete categories

Each gene involved in a polygenic trait can have multiple alleles, and the combination of these alleles can result in a wide range of phenotypes. For example, human height is a polygenic trait, and the combination of alleles from many genes can result in a wide range of heights

In addition to the additive effects of multiple genes, environmental factors can also influence the expression of polygenic traits. This interaction between genes and the environment can further increase the range of phenotypic variation

Therefore, the continuum of phenotypic variation observed in polygenic traits is a result of the combined effects of multiple genes and environmental influences

Question 16

Transmission genetics

Answer 16

The key principles of transmission genetics include:
1. Principle of Segregation: Each inherited trait is defined by a pair of genes. Offspring receive one genetic allele from each parent

2. Principle of Independent Assortment: Genes for different traits are sorted separately so that the inheritance of one trait is not dependent on the inheritance of another
3. Principle of Dominance: An organism with at least one dominant allele will display the effect of the dominant allele

Transmission genetics involves observation and explanation of phenotypic patterns both among the offspring of specified hybrid crosses and among naturally occurring families. It merges the analytical power of gene inheritance with molecular approaches, making it a valuable research tool

Question 17

Blending Inheritance

Answer 17

An outdated theory suggesting offspring inherit traits as an average of their parents’ traits. It was discarded with the acceptance of particulate inheritance

This theory implied that the genetic material of offspring was a uniform blend of that of the parents. However, blending inheritance was discarded with the general acceptance of particulate inheritance during the development of modern genetics, after around 1900

One of the main criticisms of blending inheritance was that it would lead to a loss of variation in populations, as traits would blend over generations until a uniform phenotype was reached. This is contrary to what we observe in nature, where variation is maintained over generations. Today, we understand that inheritance is particulate, not blending, thanks to the work of Gregor Mendel

Question 18

Trait

Answer 18

Part of an individual’s overall phenotype, which is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

For any given trait, one gene variation (allele) is received from the father and one from the mother. The expression of these alleles determines the phenotype, whether dominant or recessive

Question 19

Hybridization

Answer 19

In summary, hybridization is a fundamental process in genetics that involves the pairing of complementary DNA or RNA strands, the production of hybrids within a species, and the exchange of genes between species

In a broader biological context, hybridization can also refer to the process of producing a hybrid, which increases the genetic variety within a species. This is particularly important for evolution as it allows for adaptation to changing environmental conditions

Question 20

True Breeding

Answer 20

True breeding, sometimes also called a purebred or pure line, refers to an organism that always passes down certain phenotypic traits (i.e., physically expressed traits) to its offspring of many generations. This means that the parents are homozygous for every trait. In other words, they have two identical alleles for each gene

For example, a plant that has blue flowers will produce only seeds that will grow into plants that have blue flowers. With true breeding, the trait is passed on to all subsequent generations. For this to occur, the parents must be both dominant or both recessive

However, true breeding implicates a limited gene pool. As such, there is a high tendency of a particular trait to be inherited (e.g., genetic disorders) that could potentially be detrimental to the health of the offspring

Question 21

P1 generation and F1 generation

Answer 21

The P1 generation, also known as the parental generation, refers to the original set of organisms that are mated in a genetic experiment. These organisms are usually homozygous for one or more traits. This means they carry two identical alleles for each gene

The offspring of the P1 generation are known as the F1, or first filial, generation. The F1 generation can reproduce to create the F2 generation, and so forth

For example, when Gregor Mendel, the “Father of Genetics”, was studying pea genetics, he started by producing lines of pure-breeding peas. He crossed these two lines of plants, designated as the parental generation or P generation. The offspring from this cross, which were all green, were the first generation of offspring, or the F1 generation. Mendel then allowed the F1 plants to self-fertilize, producing the F2 generation

Question 22

Reciprocal Crosses

Answer 22

reciprocal cross in genetics is a pair of crosses between a male of one strain and a female of another, and vice versa. It’s used to determine whether a trait is linked to the sex chromosomes or autosomal chromosomes, and whether it’s dominant or recessive. The parents must be true breeding, and the trait is observed over two generations. This helps in understanding the inheritance pattern of the trait.

Question 23

Dominant vs Recessive

Answer 23

In genetics, dominant traits are expressed if at least one dominant allele is present. Recessive traits only show up when two recessive alleles are present. So, a person with one dominant and one recessive allele will display the dominant trait but can pass on the recessive trait to offspring.

Dominant Traits: These traits are expressed even if there is only one copy of an allele for a particular trait in the gene. For example, the allele for brown eyes is dominant, so a person will have brown eyes if they have at least one allele for brown eyes

Recessive Traits: These traits are expressed only when two copies of an allele are present in the gene. For instance, the allele for blue eyes is recessive, so a person will have blue eyes only if they have two alleles for blue eyes

In terms of inheritance, if a person receives a dominant allele from one parent and a recessive allele from the other, the dominant allele determines the characteristic. This person is considered heterozygous and is often referred to as a “carrier” of the recessive allele. If a person has two dominant alleles or two recessive alleles, they are considered homozygous

Question 24

Alleles

Answer 24

An allele is a variant form of a gene that is located at a specific position, or locus, on a chromosome. Alleles are responsible for variations in genetic traits. Here are some key points about alleles:

• Each allele is a sequence of nucleotides, which are the building blocks of DNA
• The simplest alleles are single nucleotide polymorphisms (SNPs), but they can also be insertions and deletions of up to several thousand base pairs
• Most alleles result in little or no change in the function of the gene product they code for
• Different alleles can result in different observable phenotypic traits, such as different pigmentation
• If the two alleles at a gene locus in an organism are the same, the organism is homozygous for that gene. If the alleles are different, the organism is heterozygous.
• In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive
• Some traits are determined by more than two alleles. Multiple forms of the allele may exist, though only two will attach to the designated gene site during meiosis.
• All genetic traits are the result of the interactions of alleles

Question 25

Genotype vs Phenotype

Answer 25

The genotype is the set of genes in our DNA responsible for a particular trait. The phenotype is the physical expression, or characteristics, of that trait. For example, two organisms that have even the same genotypes don’t necessarily look or act the same way because appearance and behavior are modified by environmental conditions

Question 26

F2 Generation

Answer 26

The F2 generation refers to the second filial generation that is produced by interbreeding individuals of the F1 generation. This generation consists of individuals that exhibit the result of recombination and segregation of genes controlling traits for which stocks of the P1 generation differ. In other words, the F2 generation is the offspring produced by allowing the F1 individuals to interbreed

Question 27

Homozygous

Answer 27

Homozygous refers to having inherited the same versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is homozygous for a genomic marker has two identical versions of that marker. For instance, if you have two alleles for the gene that causes brown eyes, you’re homozygous for that specific gene. You either have two dominant alleles (homozygous dominant) or two recessive alleles (homozygous recessive). Some diseases are caused by mutated alleles. If the allele is recessive, it’s more likely to cause disease in people who are homozygous for that mutated gene

Question 28

Gamete

Answer 28

A gamete is a reproductive cell or a sex cell in an organism that reproduces sexually. Gametes are haploid, meaning they contain only one set of chromosomes. There are two types of gametes:

• Male gametes, also known as sperm, are small and motile.
• Female gametes, also known as ova or egg cells, are larger and non-motile.
  During fertilization, a male and a female gamete fuse to form a new diploid organism, which contains two sets of chromosomes. The process of gamete formation involves meiosis, a type of cell division that results in four daughter cells with half the number of chromosomes of the parent cell. This ensures that when fertilization occurs, the resulting offspring will have the correct number of chromosomes

Question 29

Segregate

Answer 29

In genetics, segregation refers to the Mendelian law of segregation which states that during the formation of gametes (egg or sperm), the two copies of each gene separate so that each gamete receives only one copy. A key implication of this is that offspring thus have an equal likelihood of inheriting either one of the parental alleles. This process occurs during meiosis, a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell. This ensures that when fertilization occurs, the resulting offspring will have the correct number of chromosomes

Question 30

Principle of segregation

Answer 30

The Principle of Segregation, also known as Mendel’s First Law, states that during the formation of reproductive cells (gametes), pairs of hereditary traits (genes) for a specific characteristic separate so that offspring receive one factor from each parent. This principle explains how the genotype of offspring can be predicted based on the genotype of the parents

Key points about the Principle of Segregation:
- It was first proposed by Gregor Mendel based on his experiments with pea plants
- It applies when an organism makes gametes: each gamete receives just one gene copy, which is selected randomly
- The segregation of alleles ensures that each allele has an equal chance of being passed on to the next generation
- This principle is fundamental to our understanding of genetics and inheritance

Question 31

Zygote

Answer 31

A zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote’s genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellular organisms, the zygote is the earliest developmental stage. In humans and most other anisogamous organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring

Question 32

Heterozygous

Answer 32

Heterozygous, in genetics, refers to having inherited different versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a genomic marker has two different versions of that marker. For example, being heterozygous for hair color could mean you have one allele for red hair and one allele for brown hair. The relationship between the two alleles affects which traits are expressed. It also determines what characteristics you’re a carrier for. If the two versions are different, you have a heterozygous genotype for that gene

Question 33

Incomplete dominance vs Codominance

Answer 33

Incomplete Dominance and Codominance are two types of genetic inheritance:

• Incomplete Dominance: This occurs when the phenotype of the heterozygous organism is a blend between the phenotypes of its homozygous parents. For example, in snapdragons, a cross between a homozygous white-flowered plant and a homozygous red-flowered plant will produce offspring with pink flowers. The heterozygote phenotype is intermediate between the two homozygote phenotypes
• Codominance: This occurs when both alleles are fully expressed in the phenotype of the heterozygote. That is, no allele can block or mask the expression of the other allele. For example, individuals with blood group AB exhibit Codominance. A and B are dominant in relation to O; however, they are not dominant against each other

In summary, Incomplete Dominance results in a new phenotype that is a blend of the parent phenotypes, while Codominance results in a phenotype that simultaneously expresses the traits of both alleles

Question 34

Pleiotropy

Answer 34

Pleiotropy is a phenomenon in genetics where one gene influences multiple, seemingly unrelated phenotypic traits. This can occur due to several mechanisms such as gene pleiotropy, developmental pleiotropy, and selectional pleiotropy

• Gene Pleiotropy: Occurs when a gene product interacts with multiple other proteins or catalyzes multiple reactions
• Developmental Pleiotropy: Occurs when mutations have multiple effects on the resulting phenotype
• Selectional Pleiotropy: Occurs when the resulting phenotype has many effects on fitness (depending on factors such as age and gender)

Pleiotropy can limit the rate of multivariate evolution when natural selection, sexual selection or artificial selection on one trait favors one allele, while selection on other traits favors a different allele. Some gene evolution is harmful to an organism

Question 35

Probability

Answer 35

1. Identify the Genotypes: Determine the genotypes (genetic makeup) of the parents. For example, let’s consider a trait where the dominant allele is represented by ‘A’ and the recessive allele is represented by ‘a’. The possible genotypes for an individual could be ‘AA’ (homozygous dominant), ‘Aa’ or ‘aA’ (heterozygous), and ‘aa’ (homozygous recessive)
2. Create a Punnett Square: A Punnett square is a simple grid that allows you to determine all the possible combinations of alleles that could result from the union of the parental gametes (sex cells). Each cell within the square represents a possible genotype for an offspring
3. Fill in the Punnett Square: Place one parent’s alleles along the top of the square and the other parent’s alleles along the side. Then, fill in each cell with the combination of alleles from the corresponding row and column
4. Calculate the Probabilities: Count the number of times each genotype appears in the Punnett square. The probability of each genotype is the number of times it appears divided by the total number of cells in the square. For example, if ‘AA’ appears once in a 4-cell Punnett square, the probability of this genotype is 1/4 or 25%
5. Determine the Phenotypes: Based on the possible genotypes, you can assess the phenotypes (physical traits). For example, if allele ‘A’ is dominant and ‘a’ is recessive, then genotypes ‘AA’, ‘Aa’, and ‘aA’ will show the dominant phenotype, while ‘aa’ will show the recessive phenotype

Question 36

Addition Rule and multiplication rule

Answer 36

The Addition Rule and the Multiplication Rule are two fundamental principles in probability that are often used in genetics to predict the outcomes of genetic crosses

Addition Rule: This rule is used when calculating the probability of either of two mutually exclusive events occurring. Mutually exclusive events are events that cannot occur at the same time. In genetics, an example might be the probability of an offspring inheriting either one allele or another

Multiplication Rule: This rule is used when calculating the probability of two independent events occurring together. Independent events are events whose outcomes do not influence each other. In genetics, an example might be the probability of an offspring inheriting a specific combination of alleles from its parents

Question 37

Principle of Independent Assortment

Answer 37

The Principle of Independent Assortment, also known as Mendel’s Second Law, states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene

This principle is based on the observation that different genes are inherited independently of one another, which means they “ignore” each other when they’re sorted into gametes. This law applies when we want to predict the inheritance of two characteristics associated with two different genes

For example, consider a cross between two pea plants: one with yellow, round seeds (YYRR) and one with green, wrinkled seeds (yyrr). The F1 offspring are all yellow and round (RrYy). A cross between two of these dihybrids (or self-fertilization of a dihybrid) results in four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. These categories appear in a ratio of approximately 9:3:3:1

However, it’s important to note that this law holds true only for genes located on different chromosomes or genes far apart on the same chromosome. Genes located close together on the same chromosome tend to be inherited together due to a phenomenon called genetic linkage

Question 38

What are two observations that argue against the idea of blending inheritance?

Answer 38

The idea of blending inheritance suggests that offspring are a uniform blend of their parents’ traits. However, there are two key observations that argue against this:

1. Re-emergence of Traits: Traits can reappear in later generations after apparently disappearing. This suggests that traits are determined by discrete units of inheritance, not by a blending of the parents’ traits
2. Non-blended Offspring: Offspring can have traits that are not a blend of their parents’ traits. For example, red-flowered parents sometimes produced yellow-flowered offspring

These observations led to the conclusion that inheritance is particulate, not blending. This means that traits are controlled by discrete genetic units (genes) that are passed intact from parents to offspring

Question 39

What are the differences among gene, allele, genotype, and phenotype?

Answer 39

• Gene: A gene is a specific position along a chromosome, also called a locus. Each gene contains the information required to synthesize individual cellular components needed for survival. The coordinated expression of many different genes is responsible for an organism’s growth and activity
• Allele: An allele is a variant form of a gene. Different forms of a gene are called alleles. A diploid organism can either have two copies of the same allele or one copy each of two different alleles. Individuals who have two copies of the same allele are said to be homozygous at that locus. Individuals who receive different alleles from each parent are said to be heterozygous at that locus
• Genotype: The genotype of an organism is the combination of alleles that they possess for a specific gene. The alleles an individual has at a locus is called a genotype. The genotype of an organism is often expressed using letters
• Phenotype: The phenotype of an organism is the combination of their observable characteristics or traits. The visible expression of the genotype is called an organism’s phenotype. The phenotype is influenced by the genotype and environmental factors

Question 40

What are the genotypes and phenotypes for Mendel’s true-breeding parent plants?

Answer 40

Mendel’s true-breeding parent plants, also known as the P0 generation, were homozygous for the traits he was studying. This means their genotypes consisted of two identical alleles for each trait. For example, a plant with violet flowers might have the genotype ‘AA’, while a plant with white flowers might have the genotype ‘aa’

The phenotypes of these plants were the physical expressions of these genotypes. So, a plant with the genotype ‘AA’ would have violet flowers (phenotype), and a plant with the genotype ‘aa’ would have white flowers (phenotype)

When Mendel crossed these true-breeding plants, all of the F1 hybrid offspring were heterozygous for the contrasting trait, meaning their genotype had different alleles for the gene being examined. For instance, if a violet-flowered plant (‘AA’) was crossed with a white-flowered plant (‘aa’), all the F1 offspring would have the genotype ‘Aa’. These F1 plants would all have violet flowers, showing that the ‘A’ allele for violet flowers is dominant

Question 41

Is it possible for two individuals to have the same phenotype but different genotypes? The same genotype, but different phenotypes? How?

Answer 41

Yes, it is possible for two individuals to have the same phenotype but different genotypes, and also the same genotype but different phenotypes

• Same Phenotype, Different Genotypes: This can occur due to the presence of a dominant allele. For example, consider a trait where the dominant allele is represented by ‘A’ and the recessive allele is represented by ‘a’. The homozygous dominant (AA) and the heterozygous (Aa) genotypes will both show the same phenotype because the dominant allele ‘A’ expresses itself when present
• Same Genotype, Different Phenotypes: This can happen because the phenotype is influenced not only by the genotype but also by environmental factors. For example, identical twins share the exact same genotype, yet their phenotypes often differ in many ways, such as height, weight, and fingerprints. This is because external factors, such as nutrition and exercise, affect gene expression. Another example is that many flamingos are pink, while some are not, even if they have the same genotypes for feather color. This is because feather color depends on the flamingo’s diet

Question 42

What is the probability that any two seeds are green and two seeds are yellow in a pea pod with exactly four seeds?

Answer 42

The probability of getting two green and two yellow seeds in a pea pod with exactly four seeds depends on the genotypes of the parent plants.

If we consider a simple Mendelian trait where yellow is dominant (Y) and green is recessive (y), and both parent plants are heterozygous (Yy), then each seed has a 3/4 chance of being yellow and a 1/4 chance of being green

Question 43

What is Mendel’s two laws of inheritance

Answer 43

Mendel’s two laws of inheritance are:

Law of Segregation: This law states that every individual possesses two alleles and only one allele is passed on to the offspring. During the formation of gametes, the two copies of each gene separate so that each gamete receives only one copy

Law of Independent Assortment: This law states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene

Question 44

How do the mechanics of meiosis and the movement of homologous chromosomes underlie Mendel’s principles of segregation and independent assortment?

Answer 44

Mendel’s principles of segregation and independent assortment are based on the mechanics of meiosis. The principle of segregation is seen in Anaphase I of meiosis, where homologous chromosomes separate, ensuring each gamete receives one copy of each gene. The principle of independent assortment is observed in Metaphase I of meiosis, where the random alignment of homologous pairs leads to the independent assortment of genes found on different chromosomes. These principles explain how alleles segregate and assort independently during the formation of gametes

Question 45

How can you predict the genotypes and phenotypes of offspring if you know the genotypes of the parents?

Answer 45

1. Identify the Genotypes of the Parents: Determine the alleles that each parent possesses for the trait in question. Each parent has two alleles for each trait, which they will randomly pass on to their offspring.
2. Set Up the Punnett Square: A Punnett Square is a grid that allows you to determine the expected percentages of different genotypes in the offspring of the two parents. Write all possible allele combinations one parent can contribute to its gametes across the top of a box and all possible allele combinations from the other parent down the left side
3. Fill in the Punnett Square: Complete the genotypes in the square by filling it in with the alleles from each parent. The allele combinations along the top and sides become labels for rows and columns within the square
4. Determine Offspring Genotypes: Each box of the Punnett Square represents a possible genotype for an offspring. Since all allele combinations are equally likely to occur, a Punnett Square predicts the probability of a cross producing each genotype
5. Predict Offspring Phenotypes: You can predict the percentages of phenotypes in the offspring of this cross from their genotypes. For example, if B is dominant to b, offspring with either the BB or Bb genotype will have the dominant phenotype. Only offspring with the bb genotype will have the recessive phenotype

Question 46

A child having two heterozygous parents that carry the alleles for a recessive genetic disorder has what chance of inheriting that disorder? Explain your answer

Answer 46

If both parents are heterozygous carriers for a recessive genetic disorder, each parent has the genotype Aa, where A is the normal allele and a is the allele for the disorder.

We can use a Punnett Square to determine the probability of the child inheriting the disorder:

A a
A AA Aa
a Aa aa

From the Punnett Square, we can see that there is one possibility (aa) out of four where the child inherits the disorder. Therefore, the child has a 25% (or 1 in 4) chance of inheriting the disorder.

This is because the child must inherit the a allele from both parents to express the disorder, which is a recessive trait. If the child inherits at least one A allele, they will be a carrier (genotype Aa) but will not express the disorder because A is dominant. The child will only express the disorder if they inherit an a allele from both parents, resulting in the aa genotype. This occurs in 1 out of the 4 possibilities in the Punnett Square, hence the 25% probability.

Remember, these are probabilities, and actual outcomes can vary. Each child born to these parents has a 25% chance of inheriting the disorder, regardless of the genotypes of any previous children.

Question 47

A child having two heterozygous parents with a dominant genetic disorder has what chance of inheriting that disorder? Explain your answer.

Answer 47

If both parents are heterozygous for a dominant genetic disorder, each parent has the genotype Aa, where A is the allele for the disorder and a is the normal allele.

We can use a Punnett Square to determine the probability of the child inheriting the disorder:

A a
A AA Aa
a Aa aa
From the Punnett Square, we can see that there are three possibilities (AA, Aa, Aa) out of four where the child inherits the disorder. Therefore, the child has a 75% (or 3 in 4) chance of inheriting the disorder.

This is because the child will express the disorder if they inherit at least one A allele, which is dominant. The child will only not express the disorder if they inherit an a allele from both parents, resulting in the aa genotype. This occurs in 1 out of the 4 possibilities in the Punnett Square, hence the 25% probability of not having the disorder. Therefore, the probability of having the disorder is 100% - 25% = 75%.

Remember, these are probabilities, and actual outcomes can vary. Each child born to these parents has a 75% chance of inheriting the disorder, regardless of the genotypes of any previous children

Question 48

Explain why sex-linked traits are most often inherited by males

Answer 48

• Sex-linked traits are often inherited by males due to the structure of sex chromosomes.
• Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).
• If a gene is located on the X chromosome, males have only one copy of it, while females have two.
• For recessive X-linked traits, males are more likely to express these traits because they only have one X chromosome.
• If a male inherits a recessive allele on his X chromosome, he will express the trait because he doesn’t have a second X chromosome that could carry a dominant allele to mask the effect of the recessive one.
• Females, having two X chromosomes, will not express the recessive trait if they have a dominant allele on the other X chromosome, even if they inherit a recessive allele on one of their X chromosomes.
• Therefore, X-linked recessive disorders are much more common in males than in females

Question 49

The law of segregation states—
a. each pair of alleles segregate independently of other alleles.
b. all possible combinations of genes cannot occur in the gametes.
c. genetic traits are independent of the genes on the chromosome.
d. genes on non-homologous chromosomes assort independently of one another

Answer 49

a. Each pair of alleles segregate independently of other alleles

This law ensures that a parent, with two copies of each gene, can pass on either allele. Both alleles will have the same chance of ending up in a zygote. The law of segregation is fundamental to the principles of genetics

Question 50

______ means that the two members of the allelic pair in the zygote are the same.
a. Recessive
b. Dominant
c. Codominant
d. Heterozygote
e. Homozygous

Answer 50

e. Homozygous

In a homozygous individual, both alleles for a particular gene are identical. They can be either both dominant (e.g., AA) or both recessive (e.g., aa). This is in contrast to a heterozygous individual (Aa), where the two alleles of the gene are different

Question 51

The term genotype refers to the physical appearance, while the term phenotype refers to the
genetic makeup.
a. This is true
b. This is false

Answer 51

False

It’s flipped

Question 52

Which of the following statements is not true regarding X-linked traits.
a. Women can be carriers because they can be heterozygous for the trait
b. X-linked traits are more common in men
c. Males inherit X-linked traits from their fathers
d. Males will usually show the trait if they receive an affected X chromosome
e. All are true

Answer 52

c. Males inherit X-linked traits from their fathers.

This statement is not true. Males inherit their X chromosome from their mothers and their Y chromosome from their fathers. Therefore, males cannot inherit X-linked traits from their fathers. Instead, they inherit these traits from their mothers. The other statements are true regarding X-linked traits

Question 53

In a cross between a homozygous purple flowered plant and a homozygous white flowered
plant, all of the F1 hybrid offspring have purple flowers. What is the term for the white trait?
a. Dominant
b. Gene
c. Allele
d. Genotype
e. Recessive

Answer 53

e. Recessive

The term for the white trait in this context is Recessive. In the given scenario, the white flower color disappears in the F1 generation when crossed with a homozygous purple flowered plant, which indicates that the white color is a recessive trait. A recessive trait is one that is only expressed in the phenotype if an organism has two copies of the recessive allele. If the organism has a dominant allele, that will override the recessive one, leading to the expression of the dominant trait. In this case, purple is the dominant trait and white is the recessive trait

Question 54

In heterozygous individuals, there is/are ______ allele(s).
a. 2 identical
b. 2 different
c. 1 identical

Answer 54

b. 2 different

In heterozygous individuals, there are two different alleles. Heterozygous refers to having inherited different versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a genomic marker has two different versions of that marker. This is in contrast to a homozygous individual, where the two alleles of the gene are identical

Question 55

The law of independent assortment states that —
a. alleles of different genes separate independently of each other
b. all possible combinations of genes cannot occur in the gametes
c. genetic traits are independent of the genes on the chromosome
d. any assortment of genes is possible regardless of the parents’ genes

Answer 55

a. alleles of different genes separate independently of each other

The Law of Independent Assortment, also known as Mendel’s Second Law, states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. This law creates a large amount of variety based on different combinations of genes which have not previously occurred. However, this law applies strictly only when the genes are located on different chromosomes or are far apart on the same chromosome

Question 56

In snapdragon flowers, red is dominant (T) and white is recessive (t). What color would a
snapdragon with a genotype of Tt have if flower color was incompletely dominant?
a. Red
b. White
c. Pink
d. Both red and white
e. Not enough information

Answer 56

c. Pink

In the case of incomplete dominance, the phenotype of a heterozygous organism can actually be a blend between the phenotypes of its homozygous parents. For example, in snapdragons, a cross between a homozygous white-flowered plant and a homozygous red-flowered plant will produce offspring with pink flowers. This means that a snapdragon with a genotype of Tt (where T is for red and t is for white) would have pink flowers. This pattern of inheritance is described as incomplete dominance, meaning that neither of the alleles is completely dominant over the other: both alleles can be seen at the same time

Question 57

The mother does not have a particular genetic disorder, but the father is heterozygous for the
same dominant disorder. What is the chance of the parents passing on the trait to their
children?
a. 0%
b. 25%
c. 50%
d. 75%
e. 100%

Answer 57

c. 50%

If the father is heterozygous for a dominant disorder, his genotype is Aa, where A is the allele for the disorder and a is the normal allele. The mother does not have the disorder, so her genotype is aa.

We can use a Punnett Square to determine the probability of the child inheriting the disorder:

A a
a Aa aa
a Aa aa
From the Punnett Square, we can see that there are two possibilities (Aa, Aa) out of four where the child inherits the disorder. Therefore, the child has a 50% (or 2 in 4) chance of inheriting the disorder.

This is because the child will express the disorder if they inherit at least one A allele, which is dominant. The child will only not express the disorder if they inherit an a allele from both parents, resulting in the aa genotype. This occurs in 2 out of the 4 possibilities in the Punnett Square, hence the 50% probability of having the disorder.

Remember, these are probabilities, and actual outcomes can vary. Each child born to these parents has a 50% chance of inheriting the disorder, regardless of the genotypes of any previous children.

Question 58

In a Mendelian cross for pea plants that are heterozygote for flower color (Tt), what is the
probability that the offspring will be homozygous recessive?
a. ¼
b. ½
c. ¾
d. 1

Answer 58

a. ¼

In a Mendelian cross for pea plants that are heterozygous for flower color (Tt), the probability that the offspring will be homozygous recessive (tt) is 1/4. This is because when two heterozygous (Tt) individuals mate, the Punnett Square for the cross would look like this:

T t
T TT Tt
t Tt tt
As you can see, one out of the four squares (or 1/4) represents the homozygous recessive genotype (tt)