In article <mailman.116.1163361764.14439.mol-evol At net.bio.net>,
<drjamielove At yahoo.com> wrote:
>We often speak of humanity as having evolved through a
>bottle neck of a few dozen individuals. Perhaps, at least with regards
>to our number two chromosome, that bottle neck was one male and one
>female from the same family.
There are an inordinate number of old alleles at the Major
Histocompatibility Locus for there ever to have been such a tight
bottleneck, at least for that chromosome (6).
Fusion heterozygotes lose about 50% of their fertility, but that's
not a guarantee of extinction with a small population size--a
random mating population of 10 or 20, in simulation, has a real
chance of fixing an underdominant trait like this one that starts out
present in one copy.
Two other factors to consider: the fusion might have been fortuitously
associated with a favorable allele, or might have created one, so that
its carriers had a higher fitness to compensate for their reproductive
issues; or some genetic feature (a repeated element?) could have predisposed
to recurrent mutation to this form, the way that the 15:21 translocation
appears recurrently in the modern human population.
15:21 is actually a pretty good analog of the fusion you're describing;
it is quite harmful to heterozygote fertility, but it's present at a
noticable level in outbred humans anyway. Someone has probably done
a study on whether the various 15:21 chromosomes have a common ancestor,
though I cannot immediately lay hands on it.
Mary Kuhner mkkuhner At eskimo.com