Question

The mode of inheritance of familial adenomatous polyposis is

• A. autosomal dominant
• B. autosomal recessive
• C. X linked recessive
• D. polygenic

Hint:

Recall which tumour suppressor gene mutation underlies this polyposis syndrome and how many copies need to be abnormal.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks how familial adenomatous polyposis (FAP), a condition marked by hundreds to thousands of colorectal polyps with a near-certain risk of colorectal cancer if untreated, is passed on genetically.

Step 2: Key Concept or Approach:
FAP is caused by a mutation in the APC gene on chromosome 5q21, a tumour suppressor gene. A single mutated copy inherited from one parent is enough to produce the disease phenotype, which is the hallmark of autosomal dominant inheritance.

Step 3: Working Through the Options:
Autosomal dominant fits FAP exactly, one abnormal APC allele is sufficient, each child of an affected parent has a 50 percent chance of inheriting the condition, and both sexes are equally affected. Autosomal recessive would need two abnormal copies, which is not how APC mutations behave, and that pattern instead describes a different, much rarer polyposis condition (MUTYH-associated polyposis). X-linked recessive would show a clear male-predominant pattern with no father-to-son transmission, which is not seen in FAP. Polygenic inheritance, involving many genes each with a small effect, does not apply here since a single APC mutation drives the disease.

Step 4: Conclusion:
The correct answer is autosomal dominant.