Inheritance Patterns

Most single genes affect more than one observable trait, a phenomenon know as pleiotropy. For example, the alleles for melanin pigment affect skin color, eye color, and hair color. The ion channel gene affected in cystic fibrosis acts in the lungs, the pancreas, and other passageways, and defects cause symptoms in both these organs, as well as elsewhere in the body.

Proteins are also involved in highly ordered metabolic pathways, and a defect "upstream" can mask or prevent expression of other alleles "down-stream." This condition is known as epistasis ("standing upon"), and the upstream gene is said to be epistatic to the downstream one. For instance, on blood cells, the well-known ABO markers are actually branched sugars attached to proteins embedded in the surface of the cell. In order for the cell to attach these sugars, it must first express a gene (called fucosyltransferase) that attaches one sugar group (fucose) that is common to all blood types. Absence of functional fucosyltransferase prevents the expression of the ABO alleles.