Natural selection can maintain genetic variation by favoring heterozygous genotypes. In equatorial Africa, the best genotype to have for hemoglobin is heterozygous (HbA / HbS). In the United States, there is no advantage to the heterozygous genotype. HbA codes for normal hemoglobin, whereas HbS codes for abnormal hemoglobin that causes red blood cells to become distorted in shape (sickle cell). Explain why the heterozygous state is advantageous in Africa, but not in the United States. What has happened to the sickle-cell allele in Africa? What has happened to the sickle-cell allele in the United States?
What will be an ideal response?
Malaria is endemic in certain areas in Africa, but is not found in the United States. The heterozygous state is advantageous in equatorial Africa because heterozygous individuals are protected against the most severe effects of malaria. Although the parasite can infect the red blood cells, the infection does not progress. Homozygous dominant individuals have no problems with sickle cell disease, but are more susceptible to malaria. Homozygous recessive individuals develop sickle-cell disease and do not usually live to reproduce. The sickle-cell allele has increased over generations in Africa and is decreasing in frequency in the United States.
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