Question 1

Which of the following best explains why calico cats are almost always female?

Accepted Answer

The gene for orange and black fur is located on the X chromosome, and random X inactivation in females leads to patches of color.

Question 2

In mammals, what is the primary mechanism for dosage compensation between XX females and XY males?

Accepted Answer

Random inactivation of one X chromosome in each cell of XX individuals, forming a Barr body.

Question 3

A female mouse with genotype $$X^B X^W ($$black and white alleles) is crossed with a male mouse with genotype $$X^B Y ($$black allele). What is the expected phenotypic ratio of their offspring by sex and color?

Accepted Answer

1 black male : 1 white male : 1 black female : 1 spotted female

Question 4

What is a Barr body?

Accepted Answer

A highly condensed, inactivated X chromosome found in the nuclei of somatic cells of XX individuals.

Question 5

Which gene is responsible for initiating X chromosome inactivation in mammals, and what does it produce?

Accepted Answer

The Xist gene, which produces a long noncoding RNA that coats the X chromosome.

Question 6

In Drosophila, how is sex determined?

Accepted Answer

By the ratio of X chromosomes to the number of sets of autosomes.

Question 7

A calico female cat ($$X^B X^O) is $$crossed with an orange male cat ($$X^O Y). $$What is the expected ratio of black males, orange males, calico females, and orange females among the kittens?

Accepted Answer

1 black male : 1 orange male : 1 calico female : 1 orange female

Question 8

Which of the following statements about X inactivation is FALSE?

Accepted Answer

Once an X chromosome is inactivated in a cell, it can be reactivated during mitosis.

Question 9

In which of the following human karyotypes would you expect to see two Barr bodies in a somatic cell?

Accepted Answer

XXX

Question 10

Which step in X inactivation involves the spreading of the Xist RNA along the X chromosome?

Accepted Answer

Spreading

Question 11

A white female mouse ($$X^W X^W) is $$crossed with a black male mouse ($$X^B Y). $$What is the expected phenotypic ratio of their offspring by sex and color?

Accepted Answer

All male offspring are white; all female offspring are spotted

Question 12

Which of the following best describes the genetic mosaicism observed in female mammals?

Accepted Answer

Different cells express genes from either the maternal or paternal X chromosome due to random X inactivation.

Question 13

Which of the following is NOT a method of sex determination found in animals?

Accepted Answer

Mitochondrial DNA content

Question 14

In male Drosophila, how is dosage compensation achieved?

Accepted Answer

By upregulating the expression of X-linked genes to match the level in females.

Question 15

Which of the following best explains why a male cat is rarely calico?

Accepted Answer

A male cat would need to have two X chromosomes (XXY) to be calico, which is a rare genetic condition.

Question 16

Which of the following is TRUE regarding the maintenance phase of X inactivation?

Accepted Answer

The inactivated X chromosome remains inactive through all subsequent mitotic divisions.

Question 17

A human with the karyotype XXY would be expected to have how many Barr bodies per somatic cell?

Accepted Answer

1

Question 18

Which of the following best describes the process of X inactivation during oogenesis?

Accepted Answer

X inactivation is erased so that both X chromosomes are active for gamete formation.

Sex Determination, Dosage Compensation, and X Inactivation in Mammals

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