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?

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.