It's presence among Ashkenazi Jews is partially due to the founders effect, and -- I think -- partially due to its granting of immunity to the Black Plague.
The founders' effect is this: in any group of people, just by random chance, certain traits will be more common than in the general population. For example, in a classroom of 20 kids, it's possible that say 6 of them have red hair (30%), even though only say 3% of the general population has red hair. Now, if those kids are stranded on an island and only breed amongst themselves, then 30% of their descendants will have red hair -- despite it's rarity in the general population.
So, let's say that 2-3% of the original founding population of the Ashkenazim carried one copy of the Tay-Sachs gene, just by random chance. Since you need two copies of the gene to cause Tay-Sachs, exponentially more Jews will get Tay-Sachs than in a population where no one carries the gene.
Let's then say that carrying one copy means that you don't get the Black Plague, which kills 1/3 of Europe several times. The Black Plague kills large numbers of people who don't have the gene, meaning that the % of people who carry the gene -- and their descendants -- increases. Since the Black Plague doesn't hit the Sephardi world, they aren't affected by this population loss, and so the % of people carrying the Tay-Sachs gene doesn't increase.
There you have it.
I never knew that. When I was in Manhattan in 88 at the OB-GYN with my then wife, one of the first things the MD asked was about TS and if we were Jewish...I believe they wanted to do an amnio for a study. I told him that I thought I was pure goy or so I thought till this thread. Now, I'm convinced I'm descended from some ancient Israelite gal who was tending sheep by the Med and was swept away by a raiding party of Iberians who then traded her to some Cornish Celts around 700BC or so. LOL...whadday think?...plausible?