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

Question

Quizplus · Accepted Answer

Since the woman is normal, she must have two X chromosomes that are both normal. The man must have one X chromosome that is normal and one X chromosome that carries the gene for color blindness. 
The Punnett square for their offspring would be:

| | XN | Xn | 
|---|---|---|
|Y |XYN |XYn |
|Y |XYN |XYn |

As we can see, 1 out of 4 possible combinations would result in a color-blind child (XYn). Therefore, the probability of their first child being color-blind is 1/4 or 0.25. Thus, the correct choice is B.

Red-Green Colour Blindness Is Controlled by an X Chromosome Gene