Tay-Sachs disease is found predominately among
Ashkenazi Jews (as mentioned above), and among
French Canadians. About 1 in 30 Ashkenazi Jews, and 1 in 40 French Canadians is a carrier. Being a carrier means having a
mutated HexA gene.
These two separate groups had interbred a lot in the past (and still do), and this accounts for the fact that Tay-Sachs is found almost exclusively in these two populations. But how come it's found in two populations that are so far apart from each other? Ashkenazi Jews come from Eastern Europe, and French Canadians come from Canada (the disease is not found in France). Isn't it strange that the same disease is found in two very separate populations?
This problem bothered biologists for a long time, and particularly studiers of evolution. How could exactly the same mutation have happened on two separate occasions? This is simply statistically impossible. This is about as likely as a troupe of alligators prancing into your bedroom right now and performing a tear-jerking rendition of 'Casablanca'. But as it turns out, it makes sense after all.
The answer is that the phenotype of the disease is the same in both cases: there is no production of the Hexosaminidase A enzyme. The lack of this enzyme causes all the symptoms explained in dko's w/u. But the genotype is different. The HexA gene in Ashkenazis and in French Europeans is ruined by different mutations. So in both cases, there is some problem with the gene that causes it to stop producing Hexosaminidase A, but the mutation is different.