A woman with a family history of cystic fibrosis is assessed for her risk as a potential carrier. What is the best approximation of her risk of being a heterozygote?

• A. 0
• B. 1/2
• C. 1/4
• D. 2/3

Answer: (D)

Cystic fibrosis is an autosomal recessive disorder caused by mutations in the CFTR gene, and individuals who are carriers (heterozygotes) generally do not express the disease phenotype. When assessing the risk of being a carrier in a woman with a family history of cystic fibrosis, it is essential to understand the inheritance pattern of the disease and the genetic context.

If the woman has a family history of cystic fibrosis, it indicates that there are individuals in her family who have the disease, suggesting that both parents of an affected child are likely to be carriers. If her parents are both carriers, the woman has a probability of 2/3 of being a carrier herself. This estimation arises from the fact that she has a legitimate chance of inheriting one normal allele and one mutated allele from her parents, giving a carrier status.

In contrast, if we only considered her having only one affected sibling, the likelihood would be evaluated differently. However, based on a broader assessment considering familial context—especially if the woman's parents are known to be carriers—in this scenario, 2/3 becomes the most appropriate answer as it reflects the higher probability derived from her family history.

Thus, the reasoning behind estimating the risk of her being a heter