Tay Sach's disease is a fatal genetic condition, in which affected individuals cannot metabolize fats properly. Children who inherit the disease rarely live past the age of five. The disease is caused by the presence of a homozygous recessive gene. If a child has Tay Sach's disease, what would her parents' genotypes be?

Tay Sach's is an autosomal disease, so the child must be "aa". Therefore, both of her parents must carry the recessive gene. Neither of her parents are affected, seeing that they are old enough to reproduce (people with Tay Sach's would not live long enough to reproduce), the only possible genotype is that both parents are "Aa", or heterozygous for the dominant trait (not having Tay Sach's). There's a 25% chance that their child will inherit Tay Sach's.