Brian Berns  wrote:

> I have read many explanations of kin selection that are based on the
> "fact" that I share 1/2 of my genes with each of my parents and
> siblings, 1/8 of my genes with each of my first cousins, etc.
> 
> I have also seen it stated many times that each human shares a very
> large fraction of his/her genes (90%? 95%? 99%? -- I forget the exact
> number) with every other human.
> 
> These two claims are obviously contradictory. Can anyone resolve this
> problem for me? Thanks.
> 
> -- Brian

Others will no doubt do better, but here is my take.

"Gene" in evolution means more than just DNA. It means *differences* in
DNA. Humans might have, for example, ~99% of all their *DNA* in common,
but of the remainder, we each have 50% of the differences from each
parent.

"Relatedness" means sharing in those genes that *vary* (called alleles,
from the Greek for "other") in a population. If there are N alleles in a
population for the same gene, then your parents may each pass on one or
two of them only. This means you can be grouped in the population as
being either AA or Aa for that allele-pair, even if there are also a
range of other alleles.

Your average share in the differences in genes will be 50% or near it
from each parent. Since genes (with exceptions) split into halves at
meiosis when the sex cells (gametes) are formed, the progeny will share
genes according to the Mendelian ratio:

    1/4 AA: 1/2 Aa: 1/4 aa

for each gene/allele.

If there are around 35,000 expressed (protein-making) genes, a 1%
variation (or polytypy, as it is called) covers 3,500 genes. This
difference can be considerable. If it went to 5%, of course we are
dealing with around 17,500 genes. To make matters more complicated,
genes control the expression of other genes. So a small number of
alleles can have *major* effects in the development of a phenotype (the
structure of the organism's body over its lifetime).
-- 
John S Wilkins PhD - www.wilkins.id.au
  a little emptier, a little spent
  as always by that quiver in the self,
  subjugated, yes, and obedient.  -- Seamus Heaney
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