In: Biology
a. What is heterozygote advantage?
b. How does heterozygote advantage lead to maintenance of genetic variation?
2a.
A. heterozygote advantage:-
In a small number of cases, humans do show a heterozygote
advantage, in which the fitness of the heterozygote is superior to
either homozygote. The best known example is the β-hemoglobin locus
and its relationship to sickle cell disease. Adult hemoglobin is
composed of four polypeptides: two α chains and two β chains, coded
for by different genes. The β chain is a sequence of 146 amino
acids, with glutamic acid in position 6. This normal hemoglobin is
referred to as type A. In sickle cell disease, a mutation causes
glutamic acid to be replaced by valine at position 6 and is
referred to as hemoglobin S. Individuals who are homozygous for the
S allele (SS) have sickle cell disease. Untreated, this condition
is lethal, and affected individuals do not survive to be old enough
to have offspring.
If there were no selective advantage to the recessive allele, we
would expect it to slowly be removed from the gene pool, and the
frequency of individuals with sickle cell disease should
approximate the mutation rate of the normal A allele to the S
allele, which is extremely rare. The disease, however, is
relatively common in western and central Africa, where the
frequency of S allele can be over 15 percent. The reason for this
high frequency is due to the heterozygote AS being resistant to the
malarial parasitePlasmodium falciparum.Thus in areas where malaria
is common, AS individuals, who possess one sickling allele, have an
advantage over AA individuals, who possess none. Copies of the S
allele are lost from the gene pool when they occur in individuals
affected with sickle cell disease since they do not reproduce, but
more copiesare created in the offspring of AS individualssince they
are resistant to a severe parasitic disease, namely malaria. This
advantage compensates for the loss of individuals with sickle cell
disease, and therefore keeps the S gene at a relatively high
frequency in western and central African populations. Other
hemoglobin abnormalities, including hemoglobin C and E, also seem
to have the same effect as sickle cell disease, though the effect
is not as pronounced.Thalassemia is another hemoglobin disorder
that causes severe anemia. There are two types, α and β, which are
due to mutations at the α and β hemoglobin loci respectively. The
disease has its highest frequency in areas bordered by the
Mediterranean Sea, especially Sardinia, Greece, Cyprus, and Israel.
Similar to sickle cell disease, the disorder is fatal at an early
age and the heterozygotes are resistant to malaria
2.b
The impact ofheterozygote advantageto genetic variation in a population:-
because heterozygotes carry two different alleles, they increase the chances of those alleles appearing in thepopulation and maintaining variation.homozygotes only pass the same allele, asort of redundancy, if you will, in the population.