I was just wondering why it isn't found in the Sephardim or Oriental Jews given the fairly similar DNA ...from that study.
Is the detail in the difference rather than the similarity and why I'm curious.
Does any other ethnic group carry Tay-Sachs?
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.