Multiple Effects of Single Genes

Pleiotropy occurs when alleles at a single locus may have effects on two or more traits, influencing more than one aspect of a phenotype. A classic example is the effect of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia.

The allele HbA encodes for a normal beta hemoglobin chain. The mutant allele Hb S encodes for a defective chain. Hb S homozygotes produce only the defective hemoglobin and suffer from sickle-cell anemia.

The gene for sickle-cell anemia codes for a variant form of hemoglobin. The altered hemoglobin in turn affects the shape of the red blood cells, which clump together and block capillaries. Impaired gas flow results in damage to tissues.

At low oxygen levels, cells with only Hb S hemoglobin (Hb S /Hb S ) "sickle" and stick together. This impedes oxygen delivery and blood flow. Over time, it causes damage throughout the body.

This animation (No Audio) shows sickle shaped cells and their effects in capillaries.

This animation (Audio - Important) describes sickle-cell anemia.

REVIEW: A gene that produces multiple effects is called