A polygenic trait is one whose phenotype is influenced by more than one gene. Traits that display a continuous distribution, such as height or skin color, are polygenic. The inheritance of polygenic traits does not show the phenotypic ratios characteristic of Mendelian inheritance, though each of the genes contributing to the trait is inherited as described by Gregor Mendel. Many polygenic traits are also influenced by the environment and are called multifactorial.
A polygenic trait is a characteristic, sometimes we call them phenotypes, that are affected by many, many different genes. A classic example of this would be height. Height in humans is very strongly genetically controlled, but there are many, many different genes that control height. And this is why people are not the exact height of their parents. Skin color is another trait that is very obvious in humans that is controlled by many, many different genes. And this is why you get differences between parents and children in skin color, although they tend to resemble one another. Polygenic traits are quite different from the classical Mendelian trait in where we see that one gene controls one characteristic or one phenotype. Surprisingly, most traits in humans, and in fact most traits in most organisms, are polygenic. Mendelian traits, although we spend a lot of time talking about them, are really the exception. Most genetic characteristics or traits are controlled by many genes.- Lawrence C. Brody, Ph.D.