Some gene mutations produce characteristics so undesirable that it is
difficult to imagine any reasonable change in environmental conditions
that would make them beneficial. There are mutations that lead to the
nondevelopment of hands and feet, to the production of blood that will
not clot, to serious malformations of essential organs, and so on. Such
mutations are unqualifiedly bad.
The badness may be so severe that a fertilized ovum may be incapable of
development; or, if it develops, the fetus miscarries or the child is
stillborn; or, if the child is born alive, it dies before it matures so
that it can never have children of its own. Any mutation that brings
about death before the gene producing it can be passed on to another
generation is a _lethal mutation_.
A gene governing a lethal characteristic may be dominant. It will then
kill even though the corresponding gene on the other chromosome of the
pair is normal. Under such conditions, the lethal gene is removed in the
same generation in which it is formed.
The lethal gene may, on the other hand, be recessive. Its effect is then
not evident if the gene it is paired with is normal. The normal gene
carries on for both.
When this is the case, the lethal gene will remain in existence and
will, every once in a while, make itself evident. If two people, each
serving as a _carrier_ for such a gene, have children, a sperm cell
carrying a lethal may fertilize an egg cell carrying the same type of
lethal, with sad results.
Every species, including man, includes individuals who carry undesirable
genes. These undesirable genes may be passed along for generations, even
if dominant, before natural selection culls them out. The more seriously
undesirable they are, the more quickly they are removed, but even
outright lethal genes will be included among the chromosomes from
generation to generation provided they are recessive. These deleterious
genes make up the _genetic load_.
The only way to avoid a genetic load is to have no mutations and
therefore no gene pool. The gene pool is necessary for the flexibility
that will allow a species to survive and evolve over the eons and the
genetic load is the price that must be paid for that. Generally, the
capacity for a species to reproduce itself is sufficiently high to make
up, quite easily, the numbers lost through the combination of
deleterious genes.
The size of a genetic load depends on two factors: The rate at which a
deleterious gene is produced through mutation, and the rate at which it
is removed by natural selection. When the rate of removal equals the
rate of production, a condition of _genetic equilibrium_ is reached and
the level of occurrence of that gene then remains stable over the
generations.
Even though deleterious genes are removed relatively rapidly, if
dominant, and lethal genes are removed in the same generation in which
they are formed, a new crop of deleterious genes will appear by mutation
with every succeeding generation. The equilibrium level for such
dominant deleterious genes is relatively low, however.
Deleterious genes that are recessive are removed much more slowly. Those
persons with two such genes, who alone show the bad effects, are like
the visible portion of an iceberg and represent only a small part of the
whole. The heterozygotes, or carriers, who possess a single gene of this
sort, and who live out normal lives, keep that gene in being. If people
in a particular population marry randomly and if one out of a million is
born homozygous for a certain deleterious recessive gene (and dies of
it), one out of five hundred is heterozygous for that same gene, shows
no ill effects, and is capable of passing it on.
It may be that the heterozygote is not quite normal but does show some
ill effects—not enough to incommode him seriously, perhaps, but enough
to lower his chances slightly for mating and bearing children. In that
case, the equilibrium level for that gene will be lower than it would
otherwise be.
It may also be that the heterozygote experiences an actual advantage
over the normal individual under some conditions. There is a recessive
gene, for instance, that produces a serious disease called sickle-cell
anemia. People possessing two such genes usually die young. A
heterozygote possessing only one of these genes is not seriously
affected and has red blood cells that are, apparently, less appetizing
to malaria parasites. The heterozygote therefore experiences a positive
advantage if he lives in a region where the incidence of certain kinds
of malaria is high. The equilibrium level of the sickle-cell anemia gene
can, in other words, be higher in malarial regions than elsewhere.
Here is one subject area in which additional research is urgently
needed. It may be that the usefulness of a single deleterious gene is
greater than we may suspect in many cases, and that there are greater
advantages to heterozygousness than we know. This may be the basis of
what is sometimes called “hybrid vigor”. In a world in which human
beings are more mobile than they have ever been in history and in which
intercultural marriages are increasingly common, information on this
point is particularly important.