Question 1

Duchenne muscular dystrophy is a serious condition caused by a recessive allele of a gene on the human X chromosome. The patients have muscles that weaken over time because they have absent or decreased dystrophin, a muscle protein. They rarely live past their 20s. How likely is it for a woman to have this condition?
A) Women can never have this condition.
B) One-half of the daughters of an affected man would have this condition.
C) One-fourth of the daughters of an affected father and a carrier mother could have this condition.
D) Very rarely: it is rare that an affected male would mate with a carrier female.
E) Only if a woman is XXX could she have this condition.

Accepted Answer

A woman would need to inherit two copies of the recessive allele (one from each parent) to have the condition. This is rare because it requires an affected male to mate with a carrier female.

Question 2

Which of the following statements correctly describes linkage?
A) The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them.
B) The observed frequency of recombination of two genes that are far apart from each other has a maximum value of 100%.
C) All of the traits that Mendel studied-seed color, pod shape, flower color, and others-are due to genes linked on the same chromosome.
D) Linked genes are found on different chromosomes.

Accepted Answer

Linkage describes the tendency for genes on the same chromosome to be inherited together. The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them. This is because crossovers occur during meiosis, when homologous chromosomes pair up and exchange genetic material. If genes are located far apart on the same chromosome, there is a greater likelihood that a crossover will occur between them, separating them and allowing them to be inherited independently. Therefore, statement A correctly describes linkage.

B is incorrect because the observed frequency of recombination between two genes that are far apart is not always 100%. It is possible for occasional crossovers to occur even between genes that are far apart on the same chromosome, which would result in some recombination.

C is incorrect because not all of the traits that Mendel studied are due to genes on the same chromosome. In fact, Mendel was working with traits that assort independently, meaning they were on different chromosomes or far apart on the same chromosome.

D is also incorrect because linked genes are, by definition, located on the same chromosome, not on different chromosomes.

Question 3

In humans, clear gender differentiation occurs not at fertilization, but after the second month of gestation. What is the first event of this differentiation?
A) formation of testosterone in male embryos
B) formation of estrogens in female embryos
C) anatomical differentiation of a penis in male embryos
D) activation of the SRY gene in male embryos and development of testes
E) activation of the SRY gene in females and development of the ovaries

Accepted Answer

The first event of clear gender differentiation in humans is the activation of the SRY gene in male embryos and the subsequent development of testes, which then produce testosterone. This typically occurs around the second month of gestation.

Question 4

Red-green color blindness is a sex-linked recessive trait in humans. Two people with normal color vision have a color-blind son. What are the genotypes of the parents?
A) XNXn and XnY
B) XnXn and XNY
C) XNXN and XnY
D) XNXN and XNY
E) XNXn and XNY

Accepted Answer

The mother must be a carrier (XNXn) to pass the recessive allele to the son, and the father must have normal vision (XNY).

Question 5

Normally, only female cats have the tortoiseshell phenotype because
A) the males die during embryonic development.
B) a male inherits only one allele of the X-linked gene controlling hair color.
C) the Y chromosome has a gene blocking orange coloration.
D) only males can have Barr bodies.

Accepted Answer

The tortoiseshell phenotype is controlled by an X-linked gene, and females have two X chromosomes while males have only one. A male can inherit either the orange or black allele, but not both. Therefore, only females will have the combination of orange and black patches that create the tortoiseshell pattern. Males with the rare tortoiseshell phenotype have an extra X chromosome (XXY) or mosaicism.

Question 6

Sex determination in mammals is due to the SRY gene on the Y chromosome. An abnormality of this region could allow which of the following to have a male phenotype?
A) Turner syndrome, 45,X
B) translocation of the SRY gene to an autosome of a 46,XX individual
C) a person with an extra X chromosome
D) a person with one normal and one shortened (deleted) X

Accepted Answer

The SRY gene on the Y chromosome is responsible for male development. If this gene is translocated onto an autosome of an XX individual, that individual would develop as a male. The other options (Turner syndrome, extra X chromosome, shortened X) all result in a female phenotype.

Question 7

Map units on a linkage map cannot be relied upon to calculate physical distances on a chromosome for which of the following reasons?
A) The frequency of crossing over varies along the length of the chromosome.
B) The relationship between recombination frequency and map units is different in every individual.
C) Physical distances between genes change during the course of the cell cycle.
D) The gene order on the chromosomes is slightly different in every individual.

Accepted Answer

The frequency of crossing over varies along the length of the chromosome, meaning that the amount of physical distance between two genes may not be equal to the number of map units between them. This variation in the frequency of crossing over is due to factors such as the location of the genes on the chromosome and the presence of specific DNA sequences that affect recombination. Therefore, map units do not provide a reliable metric for calculating physical distances on a chromosome.

Question 8

Why does recombination between linked genes continue to occur?
A) Recombination is a requirement for independent assortment.
B) Recombination must occur or genes will not assort independently.
C) New allele combinations are acted upon by natural selection.
D) Without recombination there would be an insufficient number of gametes.

Accepted Answer

Recombination between linked genes can create new combinations of alleles. These new combinations can be acted upon by natural selection, potentially creating advantages for the individual carrying the new combination. This can increase the fitness of the individual and potentially lead to the spread of the new combination in the population.

Question 9

One map unit is equivalent to which of the following values?
A) the physical distance between two linked genes
B) 1% frequency of recombination between two genes
C) 1 nanometer of distance between two genes
D) the distance between a pair of homologous chromosomes
E) the recombination frequency between two genes assorting independently

Accepted Answer

One map unit (also known as one centimorgan) is defined as a 1% frequency of recombination between two genes.

Question 10

What event causes all female mammals to have one active X chromosome per cell instead of two?
A) activation of the XIST gene on the X chromosome that will become the Barr body
B) activation of the BARR gene on one X chromosome, which then becomes inactive
C) crossing over between the XIST gene on one X chromosome and a related gene on an autosome
D) inactivation of the XIST gene on the X chromosome derived from the male parent

Accepted Answer

The activation of the XIST gene on the X chromosome that will become the Barr body is the event that causes all female mammals to have one active X chromosome per cell instead of two. The XIST gene produces a RNA molecule that coats the X chromosome deriving from one parent, leading to its inactivation as a Barr body.

Question 11

What is the reason that closely linked genes are typically inherited together?
A) The likelihood of a crossover event between these two genes is low.
B) The number of genes in a cell is greater than the number of chromosomes.
C) Chromosomes are unbreakable.
D) Alleles are paired together during meiosis.
E) Genes align that way during metaphase I of meiosis.

Accepted Answer

Closely linked genes are located near each other on the same chromosome, making it less likely for a crossover event to occur between them during meiosis, thus they are typically inherited together.

Question 12

Which of the following statements best describes the SRY gene?
A) It is a gene present on the X chromosome that triggers female development.
B) It is an autosomal gene that is required for the expression of genes on the Y chromosome.
C) It is a gene region present on the Y chromosome that triggers male development.
D) It is an autosomal gene that is required for the expression of genes on the X chromosome.

Accepted Answer

The SRY gene is a gene region present on the Y chromosome that triggers male development. This gene is responsible for initiating the development of the testes, which subsequently produce hormones that help develop male characteristics.

Question 13

In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the cross of a black female and an orange male?
A) tortoiseshell females; tortoiseshell males
B) black females; orange males
C) orange females; orange males
D) tortoiseshell females; black males
E) orange females; black males

Accepted Answer

The black female is X^B X^b (where X^B is the black allele and X^b is the orange allele), and the orange male is X^o Y (where X^o is the orange allele). Their offspring will inherit one X chromosome from the mother and one from the father. 
The possible genotypes and phenotypes of the offspring are:
- X^B X^o (black female): She will have black fur because the black allele is dominant over the orange allele.
- X^B Y (black male): He will have black fur because he inherited the black allele from his mother.
- X^o Y (orange male): He will have orange fur because he inherited the orange allele from his father.
- X^B X^o (tortoiseshell female): She will have a patchwork of black and orange fur because X-linked genes undergo random X-inactivation in females during embryonic development to produce patches of cells expressing one allele or the other (the "tortoiseshell" phenotype).
Therefore, the only choice that includes tortoiseshell females and black males is D, making it the correct answer.

Question 14

When Thomas Hunt Morgan crossed his red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?
A) The gene involved is on the Y chromosome.
B) The gene involved is on the X chromosome.
C) The gene involved is on an autosome, but only in males.
D) Eye color in flies is a multifactorial trait.

Accepted Answer

The gene involved in determining eye color in flies is on the X chromosome. Since males only have one X chromosome, they will express whichever allele is present on that single chromosome. If the allele for white eyes is present on that X chromosome, the male will have white eyes. In contrast, females have two X chromosomes, so they can potentially have two different alleles for eye color.

Question 15

Which of the following events causes recombination between linked genes to occur?
A) Mutation on one homolog is different from that on the other homolog.
B) When genes are linked they always "travel" together at anaphase.
C) Crossovers between these genes result in chromosomal exchange.
D) Nonrecombinant chromosomes break and then re-join with one another.

Accepted Answer

Recombination between linked genes occurs through crossovers, which result in chromosomal exchange. This is the process of genetic recombination during which two chromosomes pair up and exchange segments of their genetic material. The crossing over of linked genes creates new gene combinations and increases genetic diversity. A mutation (A) or nonrecombinant chromosomes breaking and then rejoining (D) can also cause genetic variation, but they do not specifically result in recombination between linked genes. Linked genes do not always travel together (B) at anaphase, as they can undergo recombination events during meiosis.

Question 16

What does a frequency of recombination of 50% indicate?
A) The two genes are likely to be located on different chromosomes.
B) All of the offspring have combinations of traits that match one of the two parents.
C) The genes are located on sex chromosomes.
D) Independent assortment is hindered.

Accepted Answer

A frequency of recombination of 50% indicates that the two genes are likely to be located on different chromosomes. A recombination frequency of 50% indicates that the genes are behaving independently of each other, which is only possible if they are located on different chromosomes or very far apart on the same chromosome.

Question 17

Cinnabar eyes is a sex-linked recessive characteristic in fruit flies. If a female having cinnabar eyes is crossed with a wild-type male, what percentage of the F1 males will have cinnabar eyes?
A) 0%
B) 25%
C) 50%
D) 75%
E) 100%

Accepted Answer

The answer of Cinnabar eyes is a sex-linked recessive characteristic...

Question 18

Males are more often affected by sex-linked traits than females because
A) male hormones such as testosterone often alter the effects of mutations on the X chromosome.
B) female hormones such as estrogen often compensate for the effects of mutations on the X chromosome.
C) X chromosomes in males generally have more mutations than X chromosomes in females.
D) males are hemizygous for the X chromosome.
E) mutations on the Y chromosome often worsen the effects of X-linked mutations.

Accepted Answer

Males only have one X chromosome, making them hemizygous for X-linked traits. Therefore, any mutation on the X chromosome of males will be expressed, whereas in females, the presence of a second X chromosome can provide a normal allele to compensate for a mutated allele on the other X chromosome.

Question 19

Which of the following statements describes the chromosome theory of inheritance as expressed in the early twentieth century?
A) Individuals inherit particular chromosomes attached to genes.
B) Mendelian genes are at specific loci on the chromosome and in turn segregate during meiosis.
C) Homologous chromosomes give rise to some genes and crossover chromosomes to other genes.
D) No more than a single pair of chromosomes can be found in a healthy normal cell.
E) Natural selection acts on certain chromosome arrays rather than on genes.

Accepted Answer

The chromosome theory of inheritance as expressed in the early twentieth century states that Mendelian genes are at specific loci on the chromosome and in turn segregate during meiosis. This means that genes are carried on chromosomes and are passed down from parent to offspring through the transmission of chromosomes during meiosis.

Question 20

How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?
A) The two genes are closely linked on the same chromosome.
B) The two genes are linked but on different chromosomes.
C) Recombination did not occur in the cell during meiosis.
D) Both of the characters are controlled by more than one gene.

Accepted Answer

If the two genes are closely linked on the same chromosome, they are less likely to undergo recombination, resulting in more parental-type offspring than recombinant-type offspring in a testcross.

Quiz 12: The Chromosomal Basis of Inheritance

Which of the following statements correctly describes linkage?

In humans, clear gender differentiation occurs not at fertilization, but after the second month of gestation. What is the first event of this differentiation?

Red-green color blindness is a sex-linked recessive trait in humans. Two people with normal color vision have a color-blind son. What are the genotypes of the parents?

Normally, only female cats have the tortoiseshell phenotype because

Sex determination in mammals is due to the SRY gene on the Y chromosome. An abnormality of this region could allow which of the following to have a male phenotype?

Map units on a linkage map cannot be relied upon to calculate physical distances on a chromosome for which of the following reasons?

Why does recombination between linked genes continue to occur?

One map unit is equivalent to which of the following values?

What event causes all female mammals to have one active X chromosome per cell instead of two?

What is the reason that closely linked genes are typically inherited together?

Which of the following statements best describes the SRY gene?

In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the cross of a black female and an orange male?

When Thomas Hunt Morgan crossed his red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?

Which of the following events causes recombination between linked genes to occur?

What does a frequency of recombination of 50% indicate?

Cinnabar eyes is a sex-linked recessive characteristic in fruit flies. If a female having cinnabar eyes is crossed with a wild-type male, what percentage of the F1 males will have cinnabar eyes?

Males are more often affected by sex-linked traits than females because

Which of the following statements describes the chromosome theory of inheritance as expressed in the early twentieth century?

How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?