Polygenic traits are controlled by two or more than two genes (usually by many different genes) at different loci on different chromosomes. These genes are described as polygenes. A polygene refers to a group of genes that when expressed together produce a particular phenotype or trait. Multiple genes are involved for a particular trait to manifest. In humans, height, skin colour, and weight are determined by multiple genes that are expressed together. Polygenes allow a wide range of physical traits. For instance, height is regulated by several genes so that there will be a wide range of heights in a population. Accordingly, there are more than 400 genes at play in determining the height of an adult human apart from the non-genetic factors (e.g. nutrition) that influence the trait. The predisposition to type 2 diabetes is believed to be associated with polygenes.
Since there are several genes are at play in determining a trait, Mendelian inheritance alone may not explicate the phenotype of an organism. A phenotypic ratio where the effect of a single gene can be predicted would therefore not apply to polygene expression.
Word origin: poly ("many") + genic ("of genes")