Genetics. Tay-sachs which almost always is presents in early infancy, is due to the inheritance of a defective gene one from each parent which is termed recessive inheritance. The parents do not show the disease although testing is possible askenazic jews and french canadians have an excess of this condition and will frequently get tested before pregnancy. It is fatal and severe.
Quietly. The gene defect is carried quietly by many in some populations because they have 2 genes (sets of instruction) for each enzyme process. The people with one good and one bad gene get extra productivity from their good gene and don't end up with problems. When 2 bad genes are passed to a kid, they have no way to do the job needed to avoid problems and eventually have full blown disease.