Question

The 1-year-old boy has a sister who is phenotypically normal. What is the probability that the sister is a carrier of the disease?

(A) 100%
(B) 75%
(C) 67%
(D) 50%
(E) 33%
(F) 25%
(G) 0%

Answer 1

The probability that the sister is a carrier of the disease is 50%.

Step-by-step explanation:

In this scenario, the 1-year-old boy is affected by a recessive genetic disease. The question asks about the probability that his sister is a carrier of the disease. Since the sister is phenotypically normal, she does not show any signs of the disease, but she could still carry the recessive gene. According to genetics, when one parent is a carrier, there is a 50% chance that each of their children will also be a carrier. Therefore, the probability that the sister is a carrier of the disease is 50%.

Answer 2

The probability that the sister is a carrier of the disease can be determined based on the inheritance pattern of the disease. If the disease is autosomal recessive, and the 1-year-old boy is affected (having two copies of the recessive allele), then both of his parents must be carriers (heterozygous) for the disease allele.

In this scenario, the sister's genotype would be the key to determining her carrier status:

- If both parents are carriers (heterozygous), they can pass on either a normal allele or a disease allele to their offspring.

- If the sister is phenotypically normal, it means she must have inherited two normal alleles (one from each parent).

Therefore, the sister cannot be a carrier of the disease, and the correct answer is (G) 0%.