[ExI] biology term

[spike]

With the topic of Lee Corbin coming up, I am reminded of the last discussion
we had, which has been nearly a year ago.  I may bring up topics for the
next couple of months or more as I think of them: things he and I discussed
when together.

I don't know the terminology in genetics which is why I can't look up the
answers, but I think we have some biology hipsters here.

When a mutation is recessive and each parent carries one copy, there is
about a 25% chance the offspring will inherit both copies of the mutation.
In the case of a lot of genetic diseases, if either copy is the
non-mutation, then the disease is not expressed.  So if two carriers of that
mutation mate, then the offspring have about a 50% chance of being a carrier
and about a 25% chance of being free entirely of the mutation.

In the case of Tay-Sachs disease, it is known that some populations (such as
Ashkenazi) have a number of carriers, so the Jewish couples have a known
risk of suffering the heartbreak of a Tay-Sachs baby, who will not survive
to bring them grandchildren.  

Given that a mutation is detrimental in some cases, why does that mutation
survive in a population?  A theory that Lee Corbin and I kicked around is
that in every case where a mutation carries the risk of causing infant death
in a homozygote, there should be some health benefit somehow to the
heterozygote carriers.  In the case of Tay-Sachs, that benefit is an
increased resistance to tuberculosis.  Another example is sickle-cell anemia
gives the heterozygote carriers increased resistance to malaria.

Biology hipsters, what is that phenomenon called?  If I don't know the name
of something I could waste hours Googleing around and finding nada.  Lee
didn't know the term either.  That kind of disease, expressed only in
offspring of two carriers, should have a name.  There might be a name to the
theory that all such maladies must have some corresponding benefit,
otherwise the mutation would gradually disappear due to reduced viability of
the offspring.

spike