$$\frac{\text{Number of X chromosomes}}{\text{Number of sets of autosomes}}$$

Question 1

Which of the following best explains why individuals with Swyer syndrome (46, XY) develop as females despite having a Y chromosome?

Accepted Answer

They lack the SRY gene or have a nonfunctional SRY gene, preventing testis development.

Question 2

In humans, what is the primary determinant of male biological sex?

Accepted Answer

The presence of the SRY gene on the Y chromosome.

Question 3

A human with the karyotype 47, XXY is most likely to have which syndrome and what phenotype?

Accepted Answer

Klinefelter syndrome; phenotypically male with reduced fertility.

Question 4

In Drosophila, what is the expected sexual phenotype of an individual with the karyotype XXY and two sets of autosomes?

Accepted Answer

Female, because the X:A ratio is 1.

Question 5

Which equation correctly represents the X:A ratio used to determine sex in Drosophila?

Accepted Answer

$$\frac{	ext{Number of X chromosomes}}{	ext{Number of sets of autosomes}}$$

Question 6

Which of the following best describes dosage compensation in mammals?

Accepted Answer

One X chromosome in each female somatic cell is randomly inactivated, forming a Barr body.

Question 7

A female cat is heterozygous for an X-linked coat color gene. What is the most likely explanation for her tortoiseshell (patchy) coat pattern?

Accepted Answer

Random X-inactivation leads to mosaic expression of coat color genes.

Question 8

Which of the following is a consequence of X-inactivation in female mammals?

Accepted Answer

Females are mosaics for X-linked gene expression.

Question 9

What is the expected number of Barr bodies in a human cell with the karyotype 47, XXX?

Accepted Answer

$$N - 1 = 3 - 1 = 2$$

Question 10

Which of the following best explains why dosage compensation is necessary in species with sex chromosomes?

Accepted Answer

To balance the expression of X-linked genes between individuals with different numbers of X chromosomes.

Question 11

A human with the karyotype 45, X is most likely to have which syndrome and what phenotype?

Accepted Answer

Turner syndrome; phenotypically female with reduced fertility.

Question 12

Which gene is responsible for initiating male development in humans?

Accepted Answer

SRY gene

Question 13

In Drosophila, what would be the sexual phenotype of an individual with the karyotype XX and three sets of autosomes?

Accepted Answer

Intersex, because the X:A ratio is $$\frac{2}{3}.$$

Question 14

Which of the following statements about the pseudoautosomal regions (PARs) of the X and Y chromosomes is correct?

Accepted Answer

PARs allow pairing and recombination between X and Y chromosomes during meiosis.

Question 15

Which of the following best describes the function of the Xist gene in X-inactivation?

Accepted Answer

It produces an RNA that coats the X chromosome, leading to its inactivation.

Question 16

A male Drosophila has only one X chromosome. How does dosage compensation occur in this species?

Accepted Answer

The single X chromosome in males is hyperactivated to match the gene expression of females.

Question 17

Which of the following is a real-world consequence of X-inactivation in humans?

Accepted Answer

Heterozygous females for X-linked disorders may show mosaic symptoms.

Question 18

Which of the following karyotypes would result in a typical male phenotype in humans?

Accepted Answer

46, XY

Question 19

Which of the following mechanisms is used by mammals to achieve dosage compensation?

Accepted Answer

Random inactivation of one X chromosome in females.

Question 20

Which of the following best explains why individuals with Turner syndrome (45, X) often have phenotypic abnormalities?

Accepted Answer

They lack a second sex chromosome, leading to gene dosage imbalances.

Sex Determination and Dosage Compensation in Humans and Drosophila

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