Tay-Sachs disease
Cells continuously recycle worn-out fatty molecules, breaking them down with enzymes inside tiny recycling compartments. In Tay-Sachs disease one such enzyme is missing, so a fatty substance called GM2 ganglioside accumulates inside nerve cells, like garbage piling up when the disposal stops. This buildup is especially harmful in the brain, and in the classic infantile form it causes a relentless loss of skills and function from a few months of age.
Tay-Sachs is autosomal recessive, caused by mutations in the HEXA gene that disable the enzyme hexosaminidase A. A child must inherit a faulty copy from each parent. The disorder reaches notably higher carrier frequencies in certain populations, classically Ashkenazi Jewish communities, owing to founder effects in their history.
There is no cure for the classic form, but the high carrier rate in some groups made Tay-Sachs a landmark for carrier screening: testing prospective parents for the allele, combined with genetic counseling, has sharply reduced the number of affected births in screened communities. This entry is educational, not medical advice.
Before starting a family, two prospective parents from a high-risk background take a carrier test; both carry one HEXA mutation, and counseling explains their one-in-four recurrence risk for each pregnancy.
Carrier screening before pregnancy is one of this disease's most important legacies.
Tay-Sachs links several ideas at once: a missing enzyme (loss of function), recessive inheritance, a founder effect raising carrier frequency, and the power of community carrier screening.