Cystic fibrosis is an autosomal recessive disorder in humans. What is the probability that an offspring will have cystic fibrosis if both parents are carriers of cystic fibrosis?

Question

50%

100%

75%

25%

Answer 1

Cystic fibrosis is an autosomal recessive disorder, meaning that an individual must inherit two copies of the defective gene (one from each parent) to express the condition. If both parents are carriers (heterozygous, genotype "Ff"), we can use a Punnett square to determine the probabilities for their offspring's genotypes.

The possible combinations of alleles from the two parents are:

• FF (homozyg dominant) – not a carrier and does not have cystic fibrosis
• Ff (heterozygous) – carrier but does not have cystic fibrosis
• fF (heterozygous) – carrier but does not have cystic fibrosis
• ff (homozyg recessive) – has cystic fibrosis

With two parents that are carriers (Ff x Ff), the possible genotypes of their offspring are:

1. FF: 25% chance
2. Ff: 50% chance (25% for Ff and 25% for fF, which are genetically the same)
3. ff: 25% chance

Thus, there is a 25% probability that an offspring will have cystic fibrosis (ff).

The answer is 25%.