HYB: Heterosis

After sending my earlier message, I realized that newcomers to the list may not
know what we're talking about.  I plead guilty to the charge I'm
over-simplifying this, but I want to explain it WITHOUT resorting to equations
or probability functions.

Heterosis is a concept from population genetics, so we have to start with some
definitions.

HETEROZYGOTE	An organism with unlike members of a given set of alleles, which
consequently produces unlike gametes.

HOMOZYGOTE	An organism with like members of a given set of alleles, which
consequently produces like gametes.

In other words, heterozygotes produce surprises while homozygotes breed true for
that trait.  The population as a whole can be analyzed in terms of the frequency
of occurence of the two types.

When one of two competing alleles is unequivically superior to the other (in
terms of long-term survival value), the superior allele will eventually
eliminate the inferior allele from the population.  After that, as long as there
are no mutations (another fascinating subject)  the population will remain
homozygotic.

When each of two competing alleles has an advantage over the other under some
conditions, both tend to be maintained in equilibrium in the population.  The
result is balanced polymorphism -- a population consisting of individuals of all
three types, in a fairly constant ratio.  (Yes, it can be difficult to
distinguish between a balanced polymorphic population and one that is evolving
toward the homozygotic state.  That's the challenge of genetic statistics....)

HETEROSIS is a special case of balanced polymorphism.  Sometimes the
heterozygote is superior to both of the homozygotes.  Neither allele can
eliminate the other, but rather the population comes to consist almost
exclusively of heterozygotes.

The classic example is sickle-cell anemia.  People with the gene for sickle-cell
anemia also carry an immunity to malaria.  In the environment where these traits
developed, both homozygotic states tended to be fatal.  Children who were
homozygotic for the sickle-cell trait rarely lived to adulthood.  Some who were
homozygotic for normal cells did survive, but many succumbed to malaria.  The
heterozygotes had a significant advantage -- immunity to malaria and essentially
normal blood cells.

All this has to do with natural populations, of course.  It gets more
complicated when hybridizers start spreading pollen around and selecting
seedlings for certain traits.  


Sharon McAllister
73372.1745@compuserve.com
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