Consider the following chromosomes and if they are affected by hemophilia. X = unaffected X chromosome, x = X chromosome affected by hemophilia, and Y = Y chromosome.

If an Xx female and XY male have children, what fraction of their offspring will have an affected chromosome, and what fraction is likely to be affected by hemophilia?

(1 point)
A. 1/4 and 1/2
B. 1/2 and 1/4
C. 1/2 and 1/3
D. 1/4 and 1/4

I've been stuck on this question for a while, may someone please assist?

B. 1/2 and 1/4

Explanation:

Hemophilia is a rare disease that has to do with the inability for blood to clot. It is inherited on the X chromosome, hence, it is said to be X-linked. It is usually passed from mother to son. According to this question;

X - unaffected X chromosome

x = X chromosome affected by hemophilia

Y = Y chromosome

Therefore, in a cross between a Xx female (carrier) and a XY male (unaffected), the following chromosomes will be present in the gametes produced by each parent:

Xx- X and x gametes

XY- X and Y gametes

Using these gametes in a punnet square (see attached image), offsprings with genotypes: XX, Xx, XY and xY will be possibly produced.

XX (1) - unaffected female

Xx (1) - unaffected but carrier female

XY (1) - unaffected male

xY (1) - affected male (have hemophilia)

Based on the questions;

- 2 out of the possible 4 children will have the affected chromosomes i.e. both xY son and Xx daughter have the affected (x) chromosome. Hence, the fraction is 1/2

- 1 out of the 4 possible children is affected by hemophilia, which is the xY son. Hence, the fraction is 1/4.

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