According to Jared Diamond, author of Curse and Blessing of the Ghetto, Tay-Sachs disease is more common among Ashkenazim Eastern European Jews. Tay-Sachs started out as a mutation. Initially nature selected against the trait (it has no evolutionary significance). But as the environmental changed, nature selected for the heterozygous condition. Why?
1. Because years ago for many generations Ashkenazim Eastern European Jews were forced to live in crowed ghettos with little to no sanitation - this type of environment lead to a deadly homozygous recessive disease. Heterozygous individuals were at a better reproductive advantage.
2. Because Ashkenazim Eastern European Jews practiced a unique farming technique (slash and burn technology) which brought about the parasite which caused a deadly homozygous recessive disease. Heterozygous individuals were at a better reproductive advantage.
3. Because Ashkenazim Eastern European Jews founding populations (through the process of Founders Effect) were quite small. The mutation was already present in there population which persisted at a higher frequency due to inbreeding.
4. None of the above
5. Only a & c above