Question

Cystic fibrosis is an autosomal recessive disorder that affects lung function in humans. If a couple, who are both unaffected, have an affected child, what is the probability their next child will be an affected girl?

Answer 1

The probability of a couple who are both carriers of the cystic fibrosis gene having a next child who is an affected girl is 12.5%.

Step-by-step explanation:

Cystic fibrosis is an autosomal recessive disorder that leads to the production of thick mucus, causing problems in various organs, particularly the lungs and digestive system. If both parents are carriers (Ff) and thus unaffected, but have had an affected child (ff), it means each parent must have one normal allele (F) and one cystic fibrosis allele (f). Using a Punnett square, we can determine that the probability of them having another child with cystic fibrosis is 25% since they can only produce children with genotypes FF, Ff, Ff, or ff. Since the question asks for the probability of an affected girl, we must take into account the sex of the child. There's a 50% chance for the child to be a girl. To find the probability of the child being both affected and a girl, we multiply the probabilities of each independent event: 25% for being affected and 50% for being a girl (0.25 x 0.50 = 0.125). Therefore, the probability of the next child being an affected girl is 12.5%.