A woman is phenotypically normal but the father had the sexl

A woman is phenotypically normal, but the father had the sex-linked recessive condition of red-green colorblindness. if she has children with a man with normal vision, what is the probability that their first child with have normal vision and their second child will be colorblind?

Solution

Color blindness is expressed only at recessive condition in females which means that both alleles should be recessive, X^cX^c. But in male, single X^c confers color blindness because males have one X and one Y chromosome.

A woman who is phenotypically normal has one recessive (X^c) and one normal allele (X⁺) because her father is color blind (X^cY)

If the woman with X^cX⁺ mates with normal vision male (X⁺Y)

Probability of first child to have normal vision = 3/4 = 75%

Probability of second child to have color blindness = 1/4 = 25%