Healthy parents with one healthy child had a second child who died at the age of 5 from Tay-Sachs

Healthy parents with one healthy child had a second child who died at the age of 5 from Tay-Sachs syndrome. Determine the likelihood that the next child will also be sick.

Since the parents are healthy, and one child died of an absent disease, it should be concluded that the Tay-Sachs syndrome gene is recessive, as well as the heterozygosity of both parents for this gene. This assumption is further supported by the fact that their second child is healthy.

Let’s designate the gene that determines the development of the syndrome as c, then the gene that determines the normal development will be C.

Heterozygous parents – latent carriers of the pathological gene will be Cc, they will both produce sperm and eggs of types C and c.

All possible offspring of this married couple will look like this:

healthy children (CC, Cc) – 75%;

children with Tay-Sachs syndrome (ss) – 25%.

Answer: The probability of having a child with Tay-Sachs syndrome is 25% for this couple.