Red green color blindness is controlled by an X chromosomal gene in humans.A normal man and normal woman whose fathers are both color blind marry.What is the probability that their first child will be color blind?
A)1/2
B)1/4
C)1/8
D)1/3
E)0

Question

Quizplus · Accepted Answer

Since the woman is normal, we can assume that she has two normal X chromosomes. However, since the father is color blind, he must have one color blind X chromosome and one normal Y chromosome. Therefore, all of their male children will be color blind (since they will inherit the color blind X chromosome from their father), and none of their female children will be color blind (since they will inherit a normal X chromosome from each parent). 
The probability of their first child being male is 1/2, and the probability of that male child being color blind is 1/2. Therefore, the overall probability of their first child being a color blind male is (1/2) x (1/2) = 1/4, which is option B.

Red Green Color Blindness Is Controlled by an X Chromosomal