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In most cases, dominant–recessive inheritance is an oversimplified explanation for patterns of genetic inheritance. Incomplete dominance is a genetic inheritance pattern in which both genes influence the characteristic (Finegold, 2017). For example, consider blood type. The alleles for blood types A and B do not dominate each other. A heterozygous person with the alleles for blood type A and B will express both A and B alleles and have blood type AB.

A different type of inheritance pattern is seen when a person inherits heterozygous alleles in which one allele is stronger than the other yet does not completely dominate. In this situation, the stronger allele does not mask all of the effects of the weaker allele. Therefore, some, but not all, characteristics of the recessive allele appear. For example, the trait for developing normal blood cells does not completely mask the allele for developing sickle-shaped blood cells. About 5% of African American newborns (and relatively few Caucasians or Asian Americans) carry the recessive sickle cell trait (Ojodu, Hulihan, Pope, & Grant, 2014). Sickle cell alleles cause red blood cells to become crescent, or sickle, shaped. Cells that are sickle shaped cannot distribute oxygen effectively throughout the circulatory system (Ware, de Montalembert, Tshilolo, & Abboud, 2017). The average life expectancy for individuals with sickle cell anemia is 55 years in North America (Pecker & Little, 2018). Alleles for normal blood cells do not mask all of the characteristics of recessive sickle cell alleles, illustrating incomplete dominance. Sickle cell carriers do not develop full-blown sickle cell anemia (Chakravorty & Williams, 2015). Carriers of the trait for sickle cell anemia may function normally but may show some symptoms such as reduced oxygen distribution throughout the body and exhaustion after exercise. Only individuals who are homozygous for the recessive sickle cell trait develop sickle cell anemia.

Recessive sickle cell alleles cause red blood cells to become crescent shaped and unable to distribute oxygen effectively throughout the circulatory system. Alleles for normal blood cells do not mask all of the characteristics of recessive sickle cell alleles, illustrating incomplete dominance.

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