Question

a man inherits a high-risk brca1 gene. his sister inherits the same allele. which statement is true?
both siblings have the same probability of developing breast cancer.
both siblings will definitely develop breast cancer.
neither sibling will develop breast cancer.
only the sister will develop breast cancer.
both siblings have a greater risk of developing breast cancer relative to males and females, respectively, without brca1 mutations.

Explanation:

The BRCA1 gene mutation increases the risk of breast cancer. However, having the mutation does not guarantee cancer (so "Both siblings will definitely develop breast cancer" is wrong). Also, males can develop breast cancer (though less commonly than females), so "Only the sister will develop breast cancer" is incorrect. And "Neither sibling will develop breast cancer" is too absolute. The probability of developing cancer isn't the same for males and females (females have a higher baseline risk for breast cancer), so "Both siblings have the same probability of developing breast cancer" is wrong. But having the BRCA1 mutation does mean both have a greater risk compared to those without the mutation.

Answer:

Both siblings have a greater risk of developing breast cancer relative to males and females, respectively, without BRCA1 mutations.