Cystic fibrosis is caused by the allele f, which is completely recessive to the normal dominant allele F. Consider a couple who both have the genotype Ff. What is the chance that their second child will develop cystic fibrosis?

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The chance that their second child will develop cystic fibrosis is 25% because each child has a 1 in 4 chance of inheriting two recessive alleles (ff) when both parents are carriers (Ff). The outcome for each child is independent of the outcomes for their siblings.

Cystic Fibrosis Is Caused by the Allele F, Which Is