Describe incomplete dominance, and provide an example

Question

What will be an ideal response?

Answer 1

Incomplete dominance, a pattern of inheritance in which both alleles are expressed in the phenotype, results in a combined trait, or one that is intermediate between the two. The sickle cell trait, a heterozygous condition present in many black Africans, provides an example. Sickle cell anemia occurs in full form when a child inherits two recessive alleles. They cause the usually round red blood cells to become sickle (crescent-moon) shaped, especially under low-oxygen conditions. The sickled cells clog the blood vessels and block the flow of blood, causing intense pain, swelling, and tissue damage. Heterozygous individuals are protected from the disease under most circumstances. However, when they experience oxygen deprivation—for example, at high altitudes or after intense physical exercise—the single recessive allele asserts itself, and a temporary, mild form of the illness occurs. The sickle cell allele is common among black Africans for a special reason. Carriers of it are more resistant to malaria than are individuals with two alleles for normal red blood cells. In Africa, where malaria is common, these carriers survived and reproduced more frequently than others, leading the gene to be maintained in the black population. But in regions of the world where the risk of malaria is low, the frequency of the gene is declining.