Final answer:
The high incidence of Tay-Sachs disease in Ashkenazi Jews is best explained by pleiotropy, where one gene influences multiple traits, and it's less documented but sometimes theorized heterozygote advantage.
Step-by-step explanation:
The relatively high incidence of Tay-Sachs disease in Ashkenazi Jews demonstrates both pleiotropy and heterozygote advantage, therefore the correct answer is both a and b. Pleiotropy refers to a single gene affecting multiple traits, which is evidenced in the various symptoms that occur in Tay-Sachs disease. Heterozygote advantage, meanwhile, is a phenomenon where individuals with one copy of the recessive gene may have some form of advantage, such as resistance to certain diseases, that benefits their survival and contributes to the higher frequency of the gene in the population.
While heterozygote advantage is noted in other conditions such as sickle cell anemia, where carriers have resistance to malaria, it is not significantly documented in Tay-Sachs disease carriers. Nonetheless, the incidence among Ashkenazi Jews may be attributed to the founder effect or genetic drift rather than heterozygote advantage. The hygiene hypothesis is unrelated because it pertains to the occurrence of allergies and immune disorders in relation to reduced exposure to pathogens.