Question

1. In Drosophila again, a red-eyed heterozygous female is mated with a red-eyed male.

A) What are the phenotypes of their male offspring?

B) What is the expected ratio of those phenotypes (in their sons)?

2. A man and woman plan to have children. The woman is a hemophiliac (she suffers from hemophilia, which is an X-linked disorder), but the man is not a hemophiliac.

A) What are the chances that their sons will be hemophiliacs?

B) What are the chances that their daughters will be hemophiliacs?

C) What are the chances that their daughters will be carriers of hemophilia?

3. A married woman gives birth to a color-blind daughter. She and her husband have normal color vision. The husband happens to be a genetics student. Despite the fact that individuals with color blindness live perfectly normal lives, he is very upset by this revelation. Why? Explain your reasoning.

Answer 1

Ans(1): (A) Gene of eye colour lies on X chromosome. If female if heterozygous, it will have R⁺R genotype, and male will have genotype R⁺Y. Here, R⁺ represents the wild type red eye, R represents white eye and Y represents Y chromosome.

Now, Let we cross the parents,

Male Parent Female Parent	R+	Y
R+	R+R+ Red eyed female	R+Y Red eyed male
R	R+R Red eyed female	RY White eyed male

Hence, male offsring may have red eye as well as white eye. Any of the two possibilities may occur.

Ans(1) (B): Ratio of Red eyed sons to White eyed sons is 0.5 : 0.5. It means half of sons will have red eye and half of sons will have white eye.

Ans(2) (A): Hemophilia is a sex linked recessive disorder. Here, female parent is hemophilic , hence, have genotype xx, male parent is normal, hence, have genotype XY.

Now, let we cross the parents:

Male Parent Female Parent	X	Y
x	Xx Non-hemophilic female	xY Hemophilic male
x	Xx Non-hemophilic female	xY Hemophilic male

Now, we can see here, sons are always hemophilic. Hence, probability of a son to be hemophilic in this case = 2/2 = 1

Ans(2)(B):we can see above, daughters are never hemophilic. Hence, probability of a daughter to be hemophilic in this case = 0/2 = 0

Ans(2)(C): we can see above, both the daughters are carriers of hemophilis. Hnce probability of a daughter to be carrier of hemophilia = 2/2 = 1

Ans(3): Let we particularly talk about red-green colour blindness which is most common form of colour blindness. It is a sex linked recessive disorder. If female and male parent both are normal, they must have genotypes XX or Xx and XY respectively. Now let we do the cross,

Male Parent Female Parent	X	Y
X	Xx Non-hemophilic female	xY Hemophilic male
X or x	XX or Xx Non-hemophilic female	XY or xY Non-Hemophilic male or Hemophilic male

Now,, we can see here that daughter is always non-hemophilic, but in the question daughter is hemophilic. It means actual father of the girl child is someone else. I think, due to this reason, father is upset.