Zoe has cystic fibrosis. Which of the following is the most likely explanation for her condition?

• A. She was exposed to a viral infection that mutated her lung cells.
• B. She inherited a single dominant allele for cystic fibrosis from one parent.
• C. She inherited two defective alleles of the CFTR gene, one from each parent.
• D. She developed cystic fibrosis due to a chromosomal nondisjunction event during meiosis.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the nature of cystic fibrosis. Cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene (Cystic Fibrosis Transmembrane Conductance Regulator). This gene is responsible for producing a protein that regulates the movement of salt and water in and out of cells.
2. Step 2: Recognize the inheritance pattern of cystic fibrosis. It is an autosomal recessive disorder, meaning that an individual must inherit two defective alleles (one from each parent) to develop the condition. If only one defective allele is inherited, the individual is a carrier but does not exhibit symptoms.
3. Step 3: Evaluate the options provided in the problem. The first option, exposure to a viral infection, is incorrect because cystic fibrosis is not caused by external factors like infections but by genetic mutations. The second option, inheriting a single dominant allele, is incorrect because cystic fibrosis is recessive, not dominant. The fourth option, chromosomal nondisjunction, is incorrect because this event leads to an abnormal number of chromosomes, not specific gene mutations.
4. Step 4: Focus on the correct explanation. The third option states that Zoe inherited two defective alleles of the CFTR gene, one from each parent. This aligns with the autosomal recessive inheritance pattern of cystic fibrosis and is the most likely explanation for her condition.
5. Step 5: Conclude that understanding genetic inheritance patterns and the specific role of the CFTR gene is essential to explaining the cause of cystic fibrosis. This knowledge helps clarify why Zoe's condition is due to inheriting two defective alleles rather than other factors mentioned in the problem.