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Flashcards in Chapter 5 Deck (128):

What factors are important when trying to predict phenotype?

Dominant/recessive reslationship of alleles
Gene interactions affecting the expression of a single trait
Roles that sex and environment play


What are the four rules do genes following the mendelian inheritance pattern conform to?

1. The expresson of the genes in the offspring directly influences their traits
2. Except in mutations, genes are passed unaltered from generation to generation
3. Genes obey Mendel's law of segregation
4. Follow independent assortment.


What's a Maternal Effect?

An inheritance pattern for certain nuclear genes in which the genotype of the mother directly determines the phenotypic traits of her offspring.


Maternal effect was first studies by who and when?

Arthur Boycott


What was Arthur Boycott's Experiment?

He began with two different true-breeding strains of snails with shells or internal organs being dextral (right-handed) or sinistral (left-handed) morphology. He performed a reciprocal cross and found the offspring always followed the females morphology.


Explain why all of the offspring in the F2 generation are dextral even though some of them are dd?

The offspring are all dextral because all of the F1 mothers are Dd, and the genotype of the mother determines the phenotype of the offspring.


Who explained the unusual results found by Boycott?

Alfred Sturtevant


What did Alfred Sturtevant suggest as the reason for Boycott's results?

He suggested that snail coiling is due to a maternal effect gene that exists as a dextral or sinistral allele.


At the molecular and cellular level, how can the non-mendelian inheritance pattern of maternal effect genes be explained?

Based on the process of oogenesis in female animals. As an animal oocyte matures, many surrounding maternal cells called nurse cells provide the oocyte with nutrients and other materials.
Depending on the outcome of meiosis, the haploid oocyte may receive the D allele or the d allele. However, The surrounding nurse cells, have both and are then transported into the oocyte. Theses gene products persist after the egg has been fertilized and begins embryonic development. Thus the genotype of the mother due to the nurse cells influence the early development stages of the embryo.


If a mother is heterozygous, Dd, which gene products will the oocyte receive?

The egg cell will receive both D and d gene products.


Why is the sperm's genotype irrelevant when dealing with the maternal effect?

Because the expression of the sperm's gene would occur to late in the egg cells development.


Maternal effect genes encode proteins that are important in what?

The early steps of embryogenesis.


The accumulation of maternal gene products in the oocyte allows what?

Embryogenesis to proceed quickly after fertilization.


Maternal effect genes often play a role in what?

Cell division, cleavage pattern, and body axis orientation.


Defective alleles in maternal effect genes tend to have a dramatic effect on what?

The phenotype of the individual, altering major features of morphology, often with dire consequences.


A female that coils to the left has offspring that coil to the right. What are the genotypes of the mother of these offspring and the maternal grandmother, respectively?
a. dd, DD
b. Dd, Dd
c. dd, Dd
d. Dd, dd

d. Dd, dd


What is the molecular explanation for maternal effect?
a. the father's gene is silenced at fertilization
b. During oogenesis, nurse cells transfer gene products to the oocyte.
c. the gene products from nurse cells are needed during the very early stages of development
d. both b and c are correct.

d. both b and c are correct.


What is Epigenetic Inheritance?

is a pattern in which a modification occurs to a nuclear gene or chromosome that alters gene expression, but is not permanent over the course of many generations.


Epigenetic inheritance patterns are the results of what?

DNA and chromosomal modifications that occur during oogenesis, spermatogenesis, or early stages of embryogenesis.


Once initiated, epigenetic changes alter the expression of what?

Particular genes in a way that may be fixed during an individual's lifetime.


Do epigenetic modifications change the DNA sequence?



What is Dosage Compensation?

The phenomenon that in species with sex chromosomes, one of the sex chromosomes is altered so that males and females have similar levels of gene expression, even though they do not contain the same complement of sex chromosomes.


Who coined the term Dosage Compensation and when?

Hermann Muller


How does dosage compensation occur by female mammals?

Female mammals use the process of X-chromosome inactivation (XCI)


What is X-Chromosome inactivation?

A process in which mammals equalize the expression of x-linked genes by randomly turning off one x chromosome in the somatic cells of females.


How do Drosphilia males accomplish dosage compensation?

Doubling the expression of most X-linked genes.


Do birds use dosage compensation?

No, many Z-linked genes do not use dosage-compensation, though a rare few do.


Mary Lyon, proposed that dosage compensation in mammals occurs by what?

Inactivation of a single X chromosome in females.


What studies provided evidence to Mary Lyon and Liane Russell's proposals?

Cytoological studies.


Murray Barr and Ewart Bertram identified what?

A highly condensed structure in the interphase nuclei of somatic cells in female cats that was not found in male cats. This was known as the barr body.


What is the barr body?

A structure in the interphase nuclei of somatic cells of female mammals that is a highly condensed X chromosome.


Why is the Barr body more brightly staining in a cell nucleus than the other chromosomes?

It is very compact


Lyon suggested that both the Barr body and the calico pattern are the result of what?

X-chromosome inactivation in the cells of female mammals.


What is another name for X-chromosome inactivation?

The Lyon Hypothesis.


How does X-chromosome inactivation occur?

Intially both x chromosomes are active. However, at an early stage of embryonic development, on of the two x chromosomes is randomly inactivated in each somatic cell and becomes a barr body.
So as the cells multiply there will be patches of white and black fur because cells will have the inactive black cells or the inactive white cells.


At which stage of development does XCI initially occur?

During embryonic development.


What is the X-inactivation center?

A site on the x chromosome that appears to play a critical role in X-chromosome inactivation.


What happens if a female is missing its Xic due to a chromosome mutation?

A cell counts only one Xic and X-chromosome inactivation does not occur. Having two active x chromosome is a lethal condition for a human female embryo.


What is the function of the Tsix gene?

It is involved in choosing the x chromosome that is not inactivated.


What does Xist stand for?

X-inactive specific transcript.


What is Xist?

A gene required for compaction of the x chromosome into a barr body.


What are the three phases of XCI?



When does initiation occur in XCI?

During embryonic development


What happens during initiation in XCI?

One of the X chromosomes remains active, and the other is chosen to be inactivated.


Which of the XCI phases occurs in an adult female?

Only the maintenance phase


What happens during the spreading phase of XCI?

The chosen x chromosome is inactivated, involving the expression of the Xist gene.
The Xist RNA coats the inactivated x chromosome and recruits proteins that promote compaction.
Compaction inovlves DNA methylation and modification of histone proteins.
It is called the spreading phase because inactivation begins at the Xic and spreads in both directions along the x chromosome.


What occurs during maintenance?

The inactivated x chromosome is maintaned as a Barr body during future cell divisions.


In humans, up to __________ of x-linked genes may escape inactivation?



The genes that escape inactivation are often what?

Pseudoautosomal genes found on the x and y chromosomes


Why is dosage compensation not necessary for x-linked pseudoautosomal genes?

Because they are located on both the x and y chromosomes.


How are genes on the Barr body expressed, if they are supposed to be silenced?

The genes may be found in localized regions where the chromatin is less tightly packed and able to be transcribed.


In fruit flies, dosage compensation is achieved by
a. x-chromosome inactivation
b. turning up the expression of genes on the single x chromosome twofold in the male
c. turning down the expression of genes on the two x chromosomes to one half in the female.
d. all of the above.

b. turning up the expression of genes on the single x chromosome twofold in the male


According to the Lyon hypothesis,
a. one of the x chromosomes is converted to a Barr body in somatic cells of female mammals.
b. one of the x chromsomes is converted to a Barr body in all cells of female mammals.
c. both of the x chromosomes are converted to Barr bodies in somatic cells of female mammals.
d. Both of the x chromosomes are converted to Barr bodies in all cells of female mammals.

a. one of the x chromosomes is converted to a Barr body in somatic cells of female mammals.


Which of the following is not a phase of XCI?
a. initiation
b. spreading
c. maintenance
d. erasure

d. erasure


What is genomic imprinting?

A pattern of inheritance that involves a change in a single gene or chromosome during gamete formation. Depending on whether the modification occurs during spermatogenesis or oogenesis, imprinting governs whether an offspring will express a gene that has been inherited from its mother or father.


Genomic imprinting happens prior to what?



What does genomic fertilization involve?

A change in a single gene or chromosome during gamete formation.


What is monoallelic expression?

In the case of imprinting, refers to the phenomenon that only one of the two alleles of a given gene is transcriptionally expressed.


What would be the outcome of a cross between a heterozygous female and a male that carries two normal copies of the Igf2 gene?

All of the offspring would be normal because they would inherit an active copy from their father.


What are the three stages of imprinting?

1. the establishment of the imprint during gametogenesis
2. the maintenance of the imprint during embryogenesis and in adult somatic cells.
3. the erasure and reestablishment of the imprint in the germ cells.


Explain why the erasure phase of imprinting is necessary in eggs?

Erasure allows eggs to transmit unmethylated copies of the gene to the offspring.


Genomic imprinting is ________ in the somatic cells of an animal, but the marking of alleles can be ___________.

Permanent in the somatic cells of animals, but the marking of alleles can be altered from generation to generation.


Genomic imprinting occurs in what species?

Numerous insects
Flowering plants


Imprinting may involve what?

A single gene
A part of a chromosome
An entire chromosome
Or all of the chromsomes from one parent.


Who discovered imprinting?

Helen Crouse


What is a molecular explanation for genomic imprinting?

DNA methylation is a common way of gene regulation.
Genomic imprinting inovlves an imprinting control region (ICR) that is located near the imprinted gene. The ICR is methylated in the egg or sperm. The ICR contains binding sites for one or more proteins that regulate the transcription of the imprinted gene. Methylation causes an inhibition of transcription.


What is DNA methylation?

The phenomenon in which an enzyme covalently attaches a methyl group (-CH3) to a base (usually adenine or cytosine) in DNA.


What is the difference between maintenance methylation and de novo methylation? In what cell types (somatic cells or germline cells) do they occur?

Maintenance methylation is automatic methylation that occurs when a methylated gene replicates and is transferred to daughter cells. It occurs in somatic cells. De nova methylation is the methylation of a gene that is not already methylated. It occurs in germ-line cells.


How will people inherit Prader-Willi syndrome or Angelman syndrome.

Both PWS and AS involve a small deletion in human chromosome 15. If this delition is inherited from the mother, it leads to Angelman syndrome, in inherited from the father, it leads to Prader-Willi syndrome.


In mice, the copy of Igf2 gene that is inherited from the mother is never expressed in her offspring. This happens because the Igf2 gene from the mother
a. always undergoes a mutation that inactivates its function.
b. is deleted during oogenesis
c. is deleted during embryonic development
d. is not transcribed in the somatic cells of the offspring

d. is not transcribed in the somatic cells of the offspring


A female mouse, Igf2 Igf2-, is crossed to a male that is also Igf2 Igf2-. The expected outcome of the offspring for this cross is
a. all normal
b. all dwarf
c. 1 normal: 1 dwarf
d. 3 normal: 1 dwarf

c. 1 normal: 1 dwarf


The marking process for genomic imprinting initially occurs during
a. gametogenesis
b. fertilization
c. embryonic development
d. adulthood

b. fertilization


A female born with angelman syndrome carries a deletion in the AS gene. Which parent transmitted the deletion to her?
a. father
b. mother
c. Either mother or father

b. mother


What are nuclear genes?

Genes that are located on chromosomes found in the cell nucleus of eukaryotic cells.


Why is it called Extranuclear inheritance?

Because it is The inheritance of organellar genetic material.


What's another name for extranuclear inheritance?

cytoplasmic inheritance


What is extranuclear inheritance?

The inheritance of genetic material that is not found within the nucleus.


Who first suggested that chloroplasts contai their own DNA and when?

Yukako Chiba


The genome of the nucleoid is what?

A single circular chromosome composed of double-stranded DNA.


The nucleoid contains several copies of what?

The genome.


How is a nucleoid different from a cell nucleus?

A nucleoid is not surrounded by a membrane as is the cell nucleus.


The sizes of mitochondrial and chloroplast genomes vary greatly amont different what?



Mitochondrial genomes of animal species tend to be fairly what?



Mitochondrial genomes of fungi and protist are what size?



Mitochondrial genomes of plants tend to be what size?

Fairly large


What is the name of the DNA in mitochondria?

Mitochondrial DNA (mtDNA)


The human mtDNA carries relatively few what?



How many genes function within the mitochondrion?



mtDNA carries genes that encode what?

Robosomal RNA and transfer RNA


The rRNA and tRNA are necessary for what?

The synthesis of the 13 polypeptides that are encoded by the mtDNA.


The primary role of mitochondria is to provide cells with what?

The bulk of their adenosine triphosphate (ATP)


The 13 polypeptides are subunits of proteins that function in a process known as what?

Oxidative phosphorylation and other mitochondrial functions.


Why do mitochondria need rRNA and tRNA genes?

To translate proteins within the mitochondrial matrix.


Chloroplast genomes tend to be ________ than mitochondrial genomes, and have _______number of genes, about _______ genes.

10X Larger
110-120 genes


What do the genes in chloroplast DNA (cpDNA) encode?

rRNA and tRNA and other proteins required for photosynthesis.


Why do mtDNA and cpDNA not follow the mendelian pattern?

They are not sorted during meiosis and therefore do not segregate into gametes in the same way as nucelar chromosomes.


What is a reciprocal cross?

The sexes and phenotypes of the parents are reversed compared to a first cross.


During growth, can a patch of tissue with a white phenotype give rise to a patch with a green phenotype?

No, once a patch of tissue is white, it has lost all of the normal chloroplasts, so it could not produce green.


What is maternal inheritance?

Inheritance of DNA that occurs through the cytoplasm of the egg.


Why does maternal inheritance occurs in plants?

Because the chloroplasts are inherited only through the cytoplasm of the egg.


What is paternal leakage?

The phenomenon in which maternal inheritance is generally observed, but the male parent may, on rare occasions, provide mitochondria or chloroplasts to the zygote.


Why might mitochondrial mutations occur?

Maternally inherited
DNA damage- when more oxygen is consumed than actually used to make ATP, mitochondria produce free radicals that damage DNA.


What is heteroplasmy?

A cell contains a mixed population of mitochondria- some with disease some without.


What is endosymbiosis?

A symbiotic relationship in which the symbiont actually lives inside the larger of the two species.


What is endosymbiosis theory?

The theory that ancient orgin of plastids and mitochondria was the result of certain species of bacteria taking up residence within a primordial eukaryotic cell.


How have chloroplasts and mitochondria changed since the initial endosymbiosis events, which occured hundreds of millions of years ago?

The genomes have lost most of their genes during evolution. Many of these have been transferred to the cell nucleus


Chloroplasts were derived from what?

Cyanobacteria- A bacterial species that is capable of photosynthesis.


Mitochondria are derived from what?

Gram-negative nonsulfur purple bacteria.


Mitochondria and chloroplasts genes are similar to genes in what?



How many genes have been transfered from the mitochondrial genome to the nuclear genome of eukaryotes?



Gene transfer can also occur between what?



Extranuclear inheritance occurs due to
a. chromosomes that may become detached from the spindle during meiosis
b. genetic material that is found in chloroplasts and mitochondria
c. mutations that disrupt the integrity of the nuclear membrane
d. none of the above.

b. genetic material that is found in chloroplasts and mitochondria


A cross is made between a green four-o-clock and a varigated four o-clock. If the varigated plant provides the pollen, the expected outcome of the offspring would be
a. all plants with green leaves
b. 3 plants with green leaves to 1 plant with variegated leaves
c. 3 plants with green leaves to 1 plant with white leaves
d. some plants with green leaves, some with variegated leaves, and some with white leaves.

a. all plants with green leaves


Some human diseases are caused by mutations in mitochondrial genes. Which of the following statements is false?
a. human mitochondrial diseases follow a maternal inheritance pattern.
b. mutations associated with mitochondrial diseases often affect cells with a high demand for ATP
c. the symptoms associated with mitochondrial diseases tend to improve with age.
d. heteroplasmy plays a role in the severity of disease symptoms.

c. the symptoms associated with mitochondrial diseases tend to improve with age.


Chloroplasts and mitochondria evolved from an endosymbiotic relationship involving
a. purple bacteria and cyanobacteria, respectively
b. cyanobacteria and purple bacteria, respectively
c. cyanobacteria
d. purple bacteria.

b. cyanobacteria and purple bacteria, respectively


Describe the inheritance pattern of maternal effect genes. Expalin how the maternal effect occurs at the cellular level. What are the expected functional roles of the proteins that are encoded by maternal effect genes?

At the cellular level, this happens because maternal effect genes are expressed in diploid nurse cells and then the gene products are transported into the egg. These gene products play key roles in the early steps of embryonic development.


Drosphila embryo dies during early embryogenesis due to a recessive maternal effect allele called bicoid. The wild-type allele is designated bicoid*. What are the genotypes and phenotypes of the embryo's mother and maternal grandparents?

Mother: bic-bic-
Maternal grandmother: bic+bic-
Maternal grandfather: bic-bic+ or bic-bic-


Suppose a maternal effect gene exists as a normal dominant allele and an abnormal recessive allele. A mother who is phenotypically abnormal produces all normal offspring. Explain the genotype of the mother.

Mother is heterozygous. Her mother must have been homozygous for abnormal recessive allele. However, she inherited the normal dominant allele from her father. She produces normal offspring because this is a maternal effect gene, and the gene product of the normal dominant allele is transferred to the egg.


Explain why maternal effect genes exert their effects during the early stages of development.

The gene products are transferred from nurse cells to eggs. The gene products, mRNA and proteins, do not last a very long time before they are eventually degraded. Therefore, they can exert their effects only during the early stages of embryonic development.


With regard to the numbers of sex chromosomes, explain why dosage compensation is necessary.

It is necessary so that the balance of gene expression between the autosomes and sex chromosomes is similar between the two sexes.


Among different species, describe three distinct strategies for accomplishing dosage compensation.

In mammals, one of the x chromsomes is inactivated in females; in Drosphila, the level of transcription on the x chromsome in males is doubled; in c. elegans, the level of transcription of the x chromosome in hermaphrodites is decreased by 50% of that of males.


Describe the molecular process of x-chromsome inactivation.

It begins with the counting of Xics. If there are two x chromosomes, in the process of initiation, one is targeted for inactivation. During embryogenesis, these inactivation begins at the Xic locus and spreads to both ends of the x chromosome until it becomes a highly condensed Barr body. The Tsix gene may play a role in the choie of the x chromosome that remains active. the Xist gene, which is located in the Xic region, remains transcriptionally active on the inactivated x chromosome. It is thought to play an important role in x-chromosome inactivation by coating the inactive x chromosome. After x-chromosome inactivation is established, it is maintained in the same x chromsome in somatic cells during subsequent cell division. In germ cells, however, the x chromsomes are not inactivated, so an egg can transmit either copy of an active x chromosome.


How many Barr badies would you expect to find in humans with the following abnormal compositions of sex chromosomes.
a. XXY
b. XYY
c. XXX
X0 (a person with just a single x chromosome)

A. 1
B. 0
C. 2
D. 0


A black female cat (XbXb) and an orange cat (X0Y) were mated to each other and produced a male cat that was calico. Which sex chromosomes did this male offspring inherit from its mother and father?

Xb from its mother and X0 and y from its father.
( it is XXY)


When does the erasure and reestablishment phase of genomic imprinting occur? Why is it necessary to erase an imprint and then reestablish it in order to always maintain imprinting from the same sex of parent?

It occurs during gametogenesis. It is necessary to erase the imprint because each sex will transmit either active alleles of a gene. In somatic cells, the two alleles for a gene are imprinted according to the sex of the parent from which the allele was inherited.


How is the process of X-chromosome inactivation similar to genomic imprinting? Different?

In some species, x-chromosomes inactivation depends on the sex. this is similar to imprinting. Also, once x-chromosome inactivation occurs during embryonic development, it is remembered throughout the rest of the life of the organism, which is also similar to imprinting, It's different in that it is not sex dependent. The x chromsome that is inactivated could be inherited from the mother or the father. There was no marking process on the x chromosome that occured during gametogenesis. In contrast, genomic imprinting always involves a marking process during gametogenesis.


Extranuclear inheritance often correlates with maternal inheritance. But, paternal leakage is not uncommon. What is paternal leakage? If a cross produced 200 offspring and the rate of mitochondrial paternal leakage was 3 %, how many offspring would be expected to contain paternal mitochondria?

It is a small percentage of cases, an organelle is inherited from the paternal parent. If the allele was dominant, then 3% of the offspring would inherit the trait. About 6 offspring.


Acute murine leukemia virus (AMLV) causes leukemia in mice. This virus is easily passed from mother to offspring through the mother's milk. Describe how the formation of leukemia via AMLV resembles a maternal inheritance pattern. How could you determine that this form of leukemia is not caused by extranuclear inheritance.

Superficially, the tendency to develop this form of leukemia seems to be inherited from mother to offspring, much like the inheritance of mitochondria. To show that it is not, one could separate newborn mice from their mothers and place them with mother that do not carry AMLV. These offspring would not be expected to develop leukemia, even though their mother would.