Pleiotropy

Pleiotropy occurs when a single gene has more than one phenotypic expression. For example, the gene in pea plants that expresses the round or wrinkled texture of seeds also influences the phenotypic expressions of starch metabolism and water absorption. The allele for round seeds codes for a greater conversion of glucose to starch than does the allele for wrinkled seeds. In wrinkled seeds, then, there is more unconverted glucose. A higher concentration of glucose increases the osmotic gradient, which increases the absorption of water (by osmosis). Immature wrinkled seeds thus contain more water. When these seeds mature and dehydrate, however, the water is lost, which results in the wrinkling pattern observed. In contrast, round seeds absorb less water, lose less water during dehydration, and, as a result, wrinkle less, giving the seeds a smoother, rounder texture. As you can see, the gene for seed texture influences not only seed texture, but also starch production and water absorption.

Many disease-causing genes exhibit pleiotropy. Sickle-cell anemia, a human blood disease, is caused by an allele that incorrectly codes for hemoglobin. As a result, the abnormal hemoglobin molecule causes the red blood cell, usually circular, to become sickle shaped. In response, red blood cells do not flow through capillaries freely and oxygen is not adequately delivered throughout the body. As a result, there is a general breakdown throughout the entire body, including damage to the heart, lungs, kidneys, brain, and other organs, which in turn promotes various disorders including anemia, pneumonia, heart and kidney failure, bone abnormalities, and impaired mental functioning.