[Molecular-evolution] Re: Human Chromosome Two: Evidence of First-degree Consanguity in Human Evolution

Mary K. Kuhner via mol-evol%40net.bio.net (by mkkuhner At kingman.gs.washington.edu)
Sun Nov 12 20:05:25 EST 2006


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



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