T613: I read a really interesting book that postulated very similar things about the origins and even benefits of many recessive mutations. It’s called Survival of the Sickest. Great book and very interesting.
(Funny thing, after reading it I was talking to a doctor about it and we agreed that it was shticky, derivative and not exactly “scientific” but he confirmed that the information was scientifically founded. So I think of it as kind of Malcolm Gladwellish in that vein. Still a fascinating read, as are, admittedly, Malcolm Gladwell books.)
The fact is that there are only a few rare diseases that manifest themselves at all in carriers. Some, actually, as T613 says even benefit the carrier- according to that book, carriers for sickle cell anemia have a greater chance of surviving malaria than non-carriers.
ubiq: As everyone else said, if it is true that 1/25 are carriers and there’s a 25% chance that a child of two carriers will have Tay Sachs, then the odds of any one family (two carrier parents, 1/25 x 1/25) having a Tay Sachs child (1/4 per child) were still pretty remote on a global level. On an individual level it’s obviously terrible, which is why we test at all, but in the long run, as you picture it, there’s really not much of a difference.