Polymorphism occurs in a population when two or more distinct
forms exist without a range of phenotypes between them. Balanced
polymorphism (Gr. poly, many morphe, form) occurs
when different phenotypes are maintained at relatively stable frequencies
in the population and may resemble a population in
which disruptive selection operates.
Sickle-cell anemia results from a change in the structure of
the hemoglobin molecule. Some of the red blood cells of persons
with the disease are misshapen, reducing their ability to carry oxygen.
In the heterozygous state, the quantities of normal and sickled
cells are roughly equal. Sickle-cell heterozygotes occur in some
African populations with a frequency as high as 0.4. The maintenance
of the sickle-cell heterozygotes and both homozygous genotypes
at relatively unchanging frequencies makes this trait an example
of a balanced polymorphism.
Why hasn’t natural selection eliminated such a seemingly
deleterious gene? The sickle-cell gene is most common in regions
of Africa that are heavily infected with the malarial parasite,
Plasmodium falciparum. Sickle-cell heterozygotes are less susceptible
to malarial infections; if infected, they experience less severe
symptoms than do homozygotes without sickled cells. Individuals
homozygous for the normal allele are at a disadvantage because
they experience more severe malarial infections, and individuals
homozygous for the sickle-cell allele are at a disadvantage because
they suffer from the severe anemia that the sickle cells cause. The
heterozygotes, who usually experience no symptoms of anemia,
are more likely to survive than either homozygote. This system is
also an example of heterozygote superiority—when the heterozygote
is more fit than either homozygote. Heterozygote superiority
can lead to balanced polymorphism because perpetuation of the
alleles in the heterozygous condition maintains both alleles at a
higher frequency than would be expected if natural selection
acted only on the homozygous phenotypes.