> > El s bado, 17 de noviembre de 2012 21:42:18 UTC, Ron O �escribi :

> Humans and other vertebrates are considered to be diploids (we have
> two copies of all the genes except those on the sex chromosomes).
> Some vertebrates are recent tetraploids and the extra copies of the
> genes have not sorted themselves out.  Tetraploids have 4 copies of
> all the genes.  The common ancestor of all extant vertebrates was a
> tetraploid.  This species doubled their genome size probably over half
> a billion years ago.  This event happened so long ago that humans are
> effective diploids.  A lot of the duplicated genes have been lost over
> time.  Some of the duplicated genes still do about the same thing, but
> others have evolved to have other functions.

> All this means is that humans usually have two copies of each gene,
> one on each of the pairs of homologous chromosomes.  In many cases you
> only need one functioning copy to live, so you can have a bad mutation
> in one copy of a gene and still have a good copy to do what it should
> do.  This means that for a bad mutation to have an effect you have to
> inherit two bad copies, one from your mother and one from your
> father.  Since mutations happen in individuals you have a higher
> chance of inheriting the same bad mutation from some common ancestor
> if both breeding individuals are closely related to that common
> individual.  The only means for natural selection to remove the bad
> alleles is by mating related individuals that have the same bad
> mutation.  For recessive lethals say that you mate two carriers and
> they have 8 progeny.  By chance two of the progeny inherit two copies
> of the bad allele from both parents and they die.  They leave behind
> two sibs that have no copies of the bad allele and 4 sibs that have 1
> copy of the bad allele.  4 bad alleles have been removed from the
> population by selection.  If these two carriers mate with unrelated
> individuals that have no bad alleles there is no selection and no bad
> alleles are removed from the population because none die due to having
> two copies.  With an equivalent genetic contribution to the next
> generation 8 carriers are produced instead of just 4 carriers produced
> when the two related individuals had mated with each other.

> Mating two carriers Aa X Aa where (a) is the recessive lethal produces
> three genotypes in the ratio of 1 AA (fully normal), 2 Aa (carriers),
> and 1 aa (dead or to die before breeding).  If you mate a carrier Aa
> to a normal AA all progeny are AA normal or Aa carriers and there is
> no selection against the bad allele and it can increases in the
> population (is not removed by selection).  All this means is that if
> you don't do some inbreeding a bad allele can reach significant
> frequency in the population and not be selected against.  This
> increases the genetic load of the population over time.  So some
> inbreeding is useful to a population for keeping the genetic load low,
> but on an individual basis it is bad for the parents of the affected
> offspring.  The math tells us that the inbred matings decrease the
> frequency of bad mutations in the population.

> This is why the wood rat that never mates with close relatives can
> maintain a genetic load of 15 while other species that tolerate some
> inbreeding maintain a much lower genetic load.

> Ron Okimoto