Genotype and Phenotype

Some genetic loci are multiallelic, having more than one allele that will manifest in a variety of phenotypes. In most mammal species, for example, the immune response genes of the major histocompatibility complex are extremely polymorphic—meaning that there are many different alleles at each gene locus. The combination of alleles in each individual may result in either susceptibility or resistance to specific disease-causing agents.

The human blood group system is another example of multiple alleles resulting in many different phenotypes. The genes that determine ABO blood type encode enzymes that add particular sugar groups to proteins in blood cells. A person's specific blood type is due to the presence or absence of A and B sugar-protein complexes on the surface of red blood cells.

There are three alleles involved, A, B, and O, and six possible genotypes: AA, BB, OO, AB, AO, and BO. The various genotypes result in four different phenotypes or blood types: A, B, O, and AB. Individuals have blood type A if their genotypes are AA or AO. Individuals have blood type B if their genotypes are BB or BO. Individuals have blood type O if their genotype is OO, and they have blood type AB if their genotype is AB.

Many of these examples describe the concept of genotype and phenotype in terms of proteins or diseases that have been thoroughly analyzed by scientists. However, genotypic differences are abundant in nature and are evident in the most extraordinary ways.

A small mutation in a viral gene may make an otherwise harmless strain of the influenza virus capable of causing disease or even death. Other genetic variations in mammals, including humans, may influence aggression or other social interactions.

Some genes affect more than one unrelated characteristic. These genes are said to be pleiotropic. One gene, for example, produces melanin, which is responsible for skin pigmentation and is also involved in nerve pathways. If there is a certain mutation in the melanin gene, no melanin will be produced. In humans, this condition, called albinism, causes white skin and, usually, vision problems. In domestic cats, it causes a white-hair, blue-eye phenotype along with hearing loss.

In the past, scientists thought that pleiotropic genes were unusual. However, the Human Genome Project has shown that humans have only about one-third of the number of genes that was predicted. It is now believed that most genes are pleiotropic, serving many functions.