Question

What is the chance of a child having cystic fibrosis if one parent is affected and the other parent is a carrier?

• A. 25%
• B. 50%
• C. 70%
• D. 80%

Hint:

Cross an affected (aa) parent with a carrier (Aa) parent and count the aa offspring.

Answer 1

The Correct Option is B

Step 1: Cystic fibrosis is an autosomal recessive disorder caused by mutation of the CFTR gene. A person is affected only when both alleles are mutated. We use lowercase r for the mutant allele and R for the normal allele.

Step 2: An affected parent is rr. The other parent, being a carrier, is Rr. Cross rr with Rr: the affected parent always passes r, and the carrier passes R or r with equal chance.

Step 3: The offspring are therefore 50 percent Rr (carriers, unaffected) and 50 percent rr (affected). So the chance that a child is affected is 50 percent.

Step 4: Note the contrast: if the affected parent (rr) instead mated with a genotypically normal parent (RR), every child would be an unaffected carrier (Rr) and none would be affected. Since the question states the second parent is a carrier, the answer is 50 percent.