Each year a number of children are born with biological defects that impair normal function. One lethal disorder inherited as a recessive allele is Tay-Sachs disease. This is caused by a dysfunctional enzyme that fails to break down brain lipids of a certain class. The symptoms usually become manifest a few months after birth.
Some symptoms are seizures, blindness and degeneration of motor and mental performance.
Death is the final result of this disease in children. With Tay-Sachs disease, the brain cells of a baby are unable to metabolize gangliosides, a type of lipid, because a crucial enzyme does not work properly. As the lipids accumulate in the brain, the brain cells gradually cease to function normally. Only children who inherit two copies of the Tay-Sachs allele qualifies as a recessive. Heterozygotes lack symptoms of the disease, apparently because half the normal amount of functional enzyme is sufficient to prevent lipid accumulation in the brain.