Sex Determination & Sex Linkage

Question 1

What are sex chromosomes?

Answer 1

Chromosomes that determine the sex of an organism

In mammals, they are X and Y chromosomes.

Question 2

What is the genetic sex determination system in birds?

Answer 2

ZW system: males are ZZ, females are ZW

This system differs from the XY system in mammals.

Question 3

What is dosage compensation by X inactivation?

Answer 3

One of the X chromosomes in each female cell becomes inactivated

This results in a Barr body, which is a condensed, inactive X chromosome.

Question 4

What is the phenotypic ratio in a dihybrid cross with dominant reciprocal epistasis?

Answer 4

15:1 phenotypic ratio

This occurs when one gene masks the expression of another.

Question 5

What are the possible genotypes for a scalloped female in Drosophila crossed with a normal male?

Answer 5

X^sd X^sd x X^+ Y

F1 results will include a carrier female and a scalloped male.

Question 6

What is the expected F2 ratio from a normal female crossed with a scalloped male in Drosophila?

Answer 6

3:1 normal to scalloped, with sex bias

This reflects the sex-linked nature of the scalloped mutation.

Question 7

What does it mean for males to be hemizygous for genes on the X chromosome?

Answer 7

Males have only one copy of the X chromosome

This makes them more susceptible to X-linked recessive conditions.

Question 8

What is the role of the SRY gene?

Answer 8

It is the sex-determining region on the Y chromosome that triggers male development

SRY acts as a transcription factor activating male-specific genes.

Question 9

What is a gynandromorph?

Answer 9

An organism that has both male and female characteristics

In birds, this occurs when one side of the body has a male chromosome set and the other side a female set.

Question 10

Fill in the blank: In humans, nearly all X-linked diseases are ______.

Answer 10

recessive

Most X-linked diseases manifest more frequently in males due to their hemizygous state.

Question 11

What is the significance of the pseudoautosomal region (PAR)?

Answer 11

It contains genes shared between the X and Y chromosomes, essential for X-Y pairing during male meiosis

PARs allow for proper segregation of sex chromosomes.

Question 12

True or False: Female mammals are the heterogametic sex.

Answer 12

False

Females are the homogametic sex (XX) while males are the heterogametic sex (XY).

Question 13

What are some examples of X-linked diseases in humans?

Answer 13

• Haemophilia A (Factor XIII)
• Haemophilia B (Factor IX)
• Duchenne muscular dystrophy
• Red-green colour vision deficiency

These conditions primarily affect males due to their hemizygous nature for X-linked genes.

Question 14

What is the expected probability that a son will be color blind if his mother is a carrier and his father has normal vision?

Answer 14

0.5

This is due to the X-linked recessive inheritance pattern.

Question 15

What is a Barr body?

Answer 15

An inactivated X chromosome in female mammals

It is visible during interphase and results from X chromosome inactivation.

Question 16

What happens during random X chromosome inactivation?

Answer 16

One of the X chromosomes in a female is randomly inactivated during early development

This leads to a mosaic expression of maternal and paternal X-linked genes.

Question 17

What is the expected outcome of a cross between a white-eyed male Drosophila and a red-eyed female?

Answer 17

All F1 offspring will have red eyes

Red is dominant over white in this genetic scenario.

Question 18

What is the male-specific region of the Y chromosome (MSY)?

Answer 18

It contains genes that are primarily involved in male sex determination and spermatogenesis

The MSY is approximately 60 Mb in size.