Question

A man whose father was colour-blind marries a woman who had a colour-blind mother and normal father. What percentage of male children of this couple will be colour-blind?

• A. 25%
• B. 0%
• C. 50%
• D. 75%

Hint:

In X-linked disorders, sons get their X chromosome from their mother. Carrier mothers can pass the defective gene to 50% of sons.

Answer 1

The Correct Option is C

Step 1: Colour blindness is an X-linked recessive disorder. A man inherits his X chromosome from his mother and Y chromosome from his father.
Step 2: Since the man's father was colour-blind, and the disorder is X-linked, the man himself is not colour-blind (he received the normal X from his mother). But his genotype must be \( XY \) with a normal X.
Step 3: The woman had a colour-blind mother and normal father. So, she must be a carrier (\( X^C X \)).
Step 4: Cross between man (\( XY \)) and woman (\( X^C X \)) gives: \begin{itemize}
Male offspring: 50% \( X Y \) (normal), 50% \( X^C Y \) (colour-blind) \end{itemize} Thus, 50% of male children will be colour-blind.