On Nov 17, 4:22 pm, eridanus <leopoldo.perd
...@gmail.com> wrote:
> El s bado, 17 de noviembre de 2012 21:42:18 UTC, Ron O escribi :
> > On Nov 17, 3:22 pm, eridanus <leopoldo.perd...@gmail.com> wrote:
> > > El s bado, 17 de noviembre de 2012 20:52:18 UTC, Ron O escribi :
> > > dear Ron. Under a strict evolution philosophy, our advanced technology
> > > had permitted the survival of some genetic defects. It is were not by our
> > > advanced technology and great surpluses of food, many people with genetic
> > > disorders would had not survived. On the other hand, we tend to avoid
> > > that some of those cases could breed. But inbreeding was a problem among
> > > monarchs of Europe. Perhaps among the Kings of ancient Egypt. And in small
> > > isolated populations.
> > > Eridanus
> > Inbreeding was more general than that among humans. Humans have a low
> > genetic load compared to a lot of other species. Human geneticists
> > think that one reason why it is so low is that humans bred in small
> > family groups until recent history. The current inbreeding taboos may
> > not be that old. They were likely instituted as clans began to
> > realize who had the healthiest children once they came into contact in
> > larger social groups. The genetic load of humans is estimated to be
> > only 2.5 (2.5 lethal equivalents in everyone's genome. You should be
> > dead 2.5 times over if you were homozygous for the hundreds of
> > detrimental genes in your genome). Barnyard chickens have a genetic
> > load of around 6.0 (twice that of humans) and there is a species of
> > wood rat that has been evaluated that is an obligate outcrosser (they
> > can sense relatives and will not mate with them) that have a genetic
> > load of 15. The more inbreeding in your history the lower your
> > genetic load is expected to be under selection. As you note selection
> > intensity on humans has decreased in the last couple of centuries, and
> > we will see where that gets us.
> > Ron Okimoto
> You gave me a technical reply that looks counterintuitive at first glance.
> But as I do not understand the rational behind the technical jargon I must
> shut up. Perhaps there is a good reason behind this allusive piece of
> jargon.
> You mentioned domestic animals, even indirect domestic animals, like rats.
> Domestic animals are very selected by men. And even humans can be considered
> a special case of domestic animals. We have selected ourselves with frequent
> wars looking for some special features to be tamed and domesticated.
> Then, intuitively this concept has an aroma of something "not natural". We
> have been "selecting" humans genes in an artificial manner. Higher class
> people, were selecting people to become domesticated breeds.
> This concept is intuitive, and as I am not an expert in genetics I have not
> idea how this can be proven or disproven.
> Eridanus
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
