wrote
>
> 1) to obtain a clear explanation of the meaning of "recessive genes".
>
A recessive gene is one which only has an effect when two copies are present
in the same individual. For instance red hair is caused by a recessive gene.
If two red-haired people have children, all the children will have red hair.
If two non-redheads have children, there is a small chance that some of the
children will have red hair, if both happen to have a copy of the red hair
gene. If a red haired person marries a non-red head, usually all the
children will be non-redheads, unless the partner has a copy of the red hair
gene, in which case half the children will be redheads (all inherit one
redhair gene from the rehead parent, 50% also inherit a redhair gene from
the non-redhaired parent and are redheads, whilst 50% inherit the dominant
gene and have black or brown hair).
>
> 2) To understand how the inter-mixture of two groups (with differing
> sets of recessive genes) might benefit from their mutual exchange.
>
Many recessive genes have deleterious effects. They persist at low levels
because they can be passed on in the recessive state, where they do no
damage. However marriage to a close relative is very dangerous, because
there is a high chance of getting two copies of the same recessive.
If you have two inbred populations, then you will also get a fairly high
proportion of people with two copies of a recessive somewhere. If they
intermarry,  then the children of the first generation will be unlikely to
get two copies of a damaging recessive, and so will be healthier. (This
incidentally is why theories about "pure blood" or "racial
hygiene" are
nonsense in terms of genetics).
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