Yeah, once your kids apparently have about a 50/50 chance of coming out as retarded mutants, you know it’s time to freshen up the ol’ gene pool a little. :eek:
That’s an interesting question–one that I hope some geneticist will address. I have the distinct impression that there are simply sites on genes that are especially subject to mutation. For example, Huntington’s disease is caused by the duplication of the 3 letter codon at one site. Up to 20 or 30 duplicates (I think) don’t seem to matter, but once it goes beyond that, the disease begins. The age of onset is strongly correlated with the number of duplicates and that number tends to increase in every generation.
I had two cousins with muscular dystrophy. They both died in their late teens. Their mother is my biological aunt. There are no other cases in the extended family. Therefore it seems reasonable to suppose that it was a mutation in her genome (there were two different fathers involved) and not something she inherited. What I do not know is whether every case of MD comes from the same mutation.
Huntington’s disease exhibits what is known as “anticipation”, which means that it can get worse (start at an earlier age) with successive generations-- precisely because of that duplication process. In this particular case, though, it appears that the additional duplication process is worse when the allele comes from the father rather than from the mother. Link.
The genetic basis for MS is uncertain, and it seems more likely to be a combination of a genetic predisposition and environmental factors.
First-cousin marriages seem to have been quite acceptable among the British upper classes. There are at least a couple of P.G. Wodehouse stories where disapproving aunts or mothers wanted young girls to marry their cousins, rather than the unacceptable suitors whom they preferred.
I saw a newspaper article earlier this week about the genetic basis of MS.
Muscular dystrophy, on the other hand, is genetic.
There are several different types of muscular dystrophy.
Duchenne muscular dystrophy (which is probably what your cousins had) is an X-linked recessive genetic disorder. That means that mostly boys get it, and they get it from a mother who is a carrier- it pretty much doesn’t matter what genes they get from their fathers, since boys don’t get X chromosomes from their fathers.
How about the British royals? How inbred were they? Any more about the Habsburgs?
QE II and Philip are second cousins, once removed, via Christian IX of Denmark as a common ancestor. But wait! They’re also third cousins, as they share Queen Victoria as a great-great grandmother.
Muscular dystrophy, not MS. It is clear that MS is only weakly heritable, but MD is a sex-linked recessive that is inherited only from the mother (because a boy with it will not live long enough to breed).