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.

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