Question

Raman, who is color blind marries a normal woman, Aisha and they have a daughter, Tina (who is not color blind). If Tina marries a Ratan (normal vision), what is the probability of their first born child being a son who is color blind?

• A. 0%
• B. 25%
• C. 50%
• D. 75%
• E. 100%

Answer 1

The correct option is B 25%

Consider Raman as $X^c$Y and Aisha as XX.

Since colorblindness is X linked recessive disorder, colorblind father will pass the one copy of affected allele to daughter Tina. However, Aisha is normal. hence Tina will get normal copy of X chromosome from Aisha and will serve as carrier for the disease ($X^c$X). Carrier mother pass the affected allele to 50% son and normal X chromosome to rest 50%. Hence, out of all of her sons, 50% will be colorblind.
There is 50% (1/2) probability of having a son out of total 4 offspring every time. Hence, out of these 50% sons (1/2), 50% (1/2) will be colorblind. So, there is (1/2 x 1/2 = 1/4 or 25%) 25 % probability of having her first son as colorblind.

Therefore, the correct answer is option B.