Red-green color blindness is an X-linked recessive disorder. A woman with normal vision whose father was colorblind has children with a man with normal color vision. What is the probability that their first child will be colorblind?

A) 1/4

B) 1/2

C) 3/4

D) 0

E) 1

1/2

Explanation:

The woman is a carrier. she must have obtained the X-linked recessive allele from the father XCY. (assuming C stands for color blindness) Therefore she carries the X-linked gene for color blindness on one of her X - chromosomes as a carrier. (XCX), or (XXC)

if she is now crossed with the husband with normal vision;

then; XY vs XX

the sex and inheritance of the colorblindness is as follows

XCX - carrier girl

XX - normal girl

XCY or XY. or vice-versa (depending on which of the X chromosomes carries the C allele)

The probability depends on the sex of the child. The first child can be a boy or a girl. Assuming the first child is a boy then it can be either of the two boys so 1/2. The girls can not be colorblind because the father who should give the colorblindness X - chromosomes for XX sex-chromosomes is normal. Therefore their probability is not relevant here.