Question

A couple has four children, only one of who has sickle cell anemia. What is the probability their fifth child will be phenotypically normal?

Answer 1

Cannot be calculated. Each child's odds of having sickle cell anemia is independent of each other and depends on the parent genotypes, which we cannot calculate with certainty. However, sickle cell anemia is really only dangerous when an individual is homozygous recessive, which would be 25% of the offspring of heterozygous parents. We have this 25%, so likely the next child will have 75% odds of being phenotypically normal.

Again, we cannot be positive because we cannot prove the parents are bother heterozygous individuals.

Answer 2

I should amend my first sentence to say, "It cannot be calculated with 100% certainty."