Question

A couple went to a doctor and reported that both of them are "carriers" for a particular disorder; their first child is suffering from that disorder and that they are expecting their second child. What is the probability that the new child would be affected by the same disorder?

• A. 100%
• B. 50%
• C. 25%
• D. 75%

Answer 1

Answer: (C)

25%

According to the question, both parents are carriers and their first child is affected which means that the trait is recessive and parents are heterozygous dominant for the trait showing the normal phenotype. If it was a dominant trait, the carrier parents would have expressed it. The occurrence of their second child to be diseased is independent of the occurrence of their first child to be diseased. If genotype of both parents is Aa (heterozygous dominant), the probability of homozygous recessive genotype will be 25%. Thus each time, there is 25% probability of their child to be affected with the trait.

Gametes Aa----> Aa	A	a
A	AA normal child	Aa normal carrier child
a	Aa normal carrier child	aa affected child