In a couple, the father is normal, and mother is carrier for the gene of color blindness. What is the probability that their son will be color blind?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 50%
Colour blindness is an X-linked recessive trait. The gene is on the X chromosome. A son gets his only X from his mother, so he is colour blind if that X carries the gene. Let XC be the normal allele and Xc the colour blindness allele.
Father (normal): XCY. Mother (carrier): XCXc.
| XC (mother) | Xc (mother) | |
|---|---|---|
| XC (father) | XCXC — normal daughter | XCXc — carrier daughter |
| Y (father) | XCY — normal son | XcY — colour blind son |
Of the two kinds of sons, one is normal and one is colour blind. So the chance that a son is colour blind is 1 in 2, that is 50%. The daughters are not colour blind, because they get the normal X from the father.
NCERT Extract:
Mendelian Disorders – “… Males have only one X chromosome and females have two. The son of a woman who carries the gene has a 50 per cent chance of being colour blind. The mother is not herself colour blind because the gene is recessive. That means that its effect is suppressed by her matching dominant normal gene.”