A couple has a child with cystic fibrosis, an autosomal recessive disease. Neither parent has the disease. Another child is born, who is unaffected. What is the probability that this unaffected child is a carrier of cystic fibrosis?
The parents must be heterozygotes. They have a 1/4 chance of having a child with the disease, and a 3/4 chance of having an unaffected child. Since we know one child is unaffected, we can eliminate the homozygous recessive genotype, and we are left with only two possibilities: the child is homozygous dominant or heterozygous. If a Punnett square of the cross is examined, there is a 2/3 chance the unaffected child will be a carrier.