A family with two unaffected parents and a child with a recessive disease would like to know what the probability is that one of their unaffected children might be a carrier. What do you tell them?

Question

Quizplus · Accepted Answer

For a child to be affected by a recessive disease, both parents must be carriers (heterozygous) for the disease allele. Each child has a 25% chance of being affected (receiving one disease allele from each parent), a 50% chance of being a carrier (receiving a disease allele from one parent and a normal allele from the other), and a 25% chance of receiving two normal alleles (being unaffected and not a carrier). However, if a child is known to be unaffected, the probability distribution changes because the 25% chance of being affected is removed from consideration. This leaves a 2/3 chance (or 66.67%) that an unaffected child is a carrier (50% carrier chance out of 75% unaffected chance) and a 1/3 chance that the child is not a carrier (25% chance of two normal alleles out of 75% unaffected chance).

A Family with Two Unaffected Parents and a Child with a Recessive