Question

Infantile Tay-Sachs is a fatal disease caused by a recessive allele passed from parents to offspring. Jennifer had a baby brother who died of Infantile Tay-Sachs disease (TSD), but Jennifer did not have the disease. When she grew up and got married, her third child also died of Infantile TSD. How is this possible?

Answer 1

Jennifer and her husband were both carriers of the allele

Answer 2

The father of Jennifers child was also carrying the recessive allele for TSD, if we label the dominant A and the recessive(disease allele) it becomes clear. Jennifer will be a heterozygous Aa, carrying the allele for the disease but as she is heterozygous the allele is not expressed. Likewise the father of her children is Aa. Thus there is a 25% chance of and offspring inheriting both recessive alleles from mother and father and thus expressing the disease. This is a prime example of how recessive traits can go many generations before expressing themselves.