As a genetic counselor, you are asked to assess the risk for a couple with a family history of familial adenomatous polyposis (FAP) who are thinking about having children. Neither the husband nor the wife has colorectal cancer, but the husband has a sister with FAP. What is the probability that this couple will have a child with FAP? Are there any tests that you could recommend to help in this assessment?

Verified step by step guidance

1. insert step 1: Understand the genetic basis of familial adenomatous polyposis (FAP). FAP is an autosomal dominant disorder caused by mutations in the APC gene. This means that only one copy of the mutated gene is needed for an individual to be affected.
2. insert step 2: Analyze the family history. The husband's sister has FAP, indicating that one of the husband's parents likely carries the mutated APC gene. Since the husband does not have FAP, he likely did not inherit the mutated gene.
3. insert step 3: Determine the husband's genotype. Since FAP is autosomal dominant and the husband does not have the condition, he is likely homozygous for the normal APC gene (i.e., he does not carry the mutation).
4. insert step 4: Assess the risk for the couple's children. If the husband does not carry the mutated gene, he cannot pass it on to his children. Therefore, the risk of their children having FAP is negligible.
5. insert step 5: Recommend genetic testing. To confirm the husband's genotype, genetic testing for the APC mutation can be performed. This will provide a definitive answer regarding his carrier status and further inform the risk assessment for their children.

Key Concepts

Familial Adenomatous Polyposis (FAP)

Familial Adenomatous Polyposis (FAP) is a hereditary condition characterized by the development of numerous polyps in the colon and rectum, which significantly increases the risk of colorectal cancer. It is caused by mutations in the APC gene, and individuals with FAP typically have a 100% risk of developing colorectal cancer if untreated. Understanding FAP is crucial for assessing genetic risks in families with a history of the condition.

Autosomal Dominant Inheritance

FAP follows an autosomal dominant inheritance pattern, meaning that only one copy of the mutated gene from an affected parent can lead to the condition in offspring. In this case, the husband’s sister has FAP, indicating that the husband may carry the mutation. This concept is essential for calculating the probability of the couple's children inheriting the condition, as each child has a 50% chance of inheriting the mutated gene if the father is a carrier.

Genetic Testing and Counseling

Genetic testing can identify mutations in the APC gene, providing critical information for assessing the risk of FAP in the couple's children. Genetic counseling can help the couple understand their options, including testing for the husband to determine if he carries the mutation. This process aids in informed decision-making regarding family planning and potential surveillance strategies for early detection of colorectal cancer.