Features

• An organism's observable characteristics are called its phenotype, while the genes it inherited are called its genotype. Your hair color, for example, is part of your phenotype. Simple traits that can be attributed to a single gene commonly display discrete phenotypes. The pea plants studied by Mendel, for example, either had red flowers, white flowers or pink flowers; they did not exhibit a range of colors in between. Polygenic characteristics, by contrast, typically display a continuous range of phenotypes that differ from each other only slightly.
  
  

Patterns

• Polygenic characteristics are determined by the cumulative effect of many different genes. Human height, for example, displays a much more continuous range of variation than Mendel's flowers; some people are 5 foot 11, others are 5 foot 10, still others are 5 foot 10 and a half. and so forth. This kind of continuous variability is the hallmark of a polygenic characteristic. These kinds of traits are also called quantitative traits.
  
  

Considerations

• Each one of the individual genes that determines a polygenic trait individually is passed on, in keeping with Mendel's laws of inheritance. Inheritance of polygenic traits, however, can be very complicated, because some of the genes may be linked (i.e. so close to each other on a chromosome that they are often inherited together), some may exhibit epistatic effects (i.e. they may mask or modify the effect of other genes) and because many different genes are involved.
  
  

Other Factors

• To make matters worse, many polygenic characteristics are not determined by genes alone. They may also be affected by environmental factors. Your diet as a child, for example, might help determine what height you attain in addition to the genes you inherited from your parents. Often there are multiple genotypes that correspond to a single phenotype. Consequently, analysis of polygenic inheritance is very complicated and still presents a difficult problem for geneticists, especially in the human population.