Sickle cell anemia is a hereditary disease that affects the hemoglobin
molecules in the blood. The disease occurs because one amino acid changes
in the hemoglobin molecule. The result is a sickle shaped red blood cell.
The change happens close to the oxygen binding site on the hemoglobin
molecule. If there is oxygen in the blood things go all right but when
there is not enough oxygen the changed hemoglobin molecules stick together
in clumps.
Individuals who have one normal (s) allele and one sick (S') allele are
heterozygous (sS') carriers of the disease. Since neither allele is dominant
in this case, they produce both normal and sickle shaped blood cells.
People who have two alleles with the (S'S') sick allele rarely reproduce.
Sickle cell anemia is common in the African-American population in the
United States and Canada. Out of 100 African-Americans 10 are carriers
but will not be sick because they are heterozygous (Ss) for the allele.
One out of 100 African-Americans is homozygous and will become sick with
sickle cell anemia. In some African populations, especially in Central
Africa almost 45 % of the population (45 people out of 100) carry one
allele for the sickle cell trait. Why is it that the heterozygous form
of sickle cell anemia is so common in Central Africa even though the homozygous
individuals die without reproducing? The answer has to do with environmental
conditions and the presence of malaria in Central Africa.
Since the heterozygous individuals rarely get malaria they survive better
than those individuals who have the "wild" type (ss) genotype
and also survive better than those who have the homozygous dominant (SS)
genotype since these individuals die young. The presence of the malaria
parasite in the environment causes natural selection and maintains the
heterozygous (Ss) population.
«««
Source: Your Health Encyclopedia, 4-rd Edition, 2002
Last Revised at December 6, 2007 by Amalia K. Gagarina, M.S., R.D.
|