12.8: Polygenic Traits
When more than one gene is responsible for a given phenotype, the trait is considered polygenic. Human height is a polygenic trait. Studies have uncovered hundreds of loci that influence height, and there are believed to be many more. Due to the high number of genes involved, as well as environmental and nutritional factors, height varies significantly within a given population. The distribution of height forms a bell-shaped curve, with relatively few individuals in the population at the minimum or maximum heights and the majority of the population in the middle height range. Most polygenic traits, like weight, blood pressure, and aspects of fingerprint patterns, also plot as bell-shaped curves.
Human Height—a Polygenic Trait
Although Mendel’s seminal work on genetic inheritance focused on traits that arose from single genes, experiments such as genome-wide association studies have revealed that many human traits develop through the cooperation of multiple gene products. The collaboration of numerous genes to influence a phenotype constitutes a polygenic (i.e., “many gene”) trait.
One example of a polygenic trait is human height. Hundreds of loci are implicated in human height variability, and it is believed that there are more that have not yet been identified. Many of these genes directly or indirectly affect cartilage in growth plates, which are found in the long bones of the arms and legs.
Population Distribution of Polygenic Traits
Human height varies considerably, in part due to a large number of genes that influence it. Height is also affected by environmental and nutritional factors, such as whether or not the mother smoked during pregnancy, the mother’s diet during pregnancy, and the offspring’s diet.
In a population in which the minimum female height is approximately five feet, and the maximum female height is about six feet, women of all heights in between these values are present. Plotting the height distribution results in a bell-shaped curve, with relatively few women approaching the minimum or maximum heights and the majority exhibiting heights near the average of five-and-a-half feet. Most polygenic traits, such as weight, blood pressure, and aspects of fingerprint patterns, like the number of ridges, also take on bell-shaped distributions due to the high number of possible combinations of alleles.