A pregnant woman knows that she and her husband are both carriers of a recessive cystic fibrosis mutation and have a 25% chance of having a child with cystic fibrosis. The couple already has one child who suffers from cystic fibrosis. They are shocked to discover their second child also has cystic fibrosis. Explain how this couple could have two children who both exhibit the homozygous recessive disease phenotype.