Polygenic inheritance occurs when one characteristic is controlled by two or more genes (usually by many different genes) at different loci on different chromosomes. These genes are described as polygenes. Examples of human polygenic inheritance are height, skin colour and weight.
Each dominant allele has a small quantitative effect individually on the phenotype and these allelic effects are additive.
The frequency of the phenotypes of these traits follows a normal continuous variation distribution pattern. This shows that there are many possible types of allelic combination. When the values are plotted, a bell-shaped curve is obtained. The more genes are involved, the more continuous the variation and the distribution of the phenotypes.
Many human traits which are controlled by polygenes can be influenced to some extent by environmental factors. Examples include height in humans. Human height is determined by polygene, the different genes located at different chromosomes. Each of these genes has its own number of alleles. There is therefore a large number of possible combinations of alleles. Each gene has a small effect on height. The overall effect will depend on the additive effects of all the different alleles of the polygenes in a person's cells. As there are many different genes and many different alleles, there are many different possible combinations. These different combinations will produce people with many different heights. The result is continuous variation. Variation in human height is caused by genotype and can be influenced by the environment. For example, if a person has inherited a number of dominant alleles for tallness, he has the potential to grow tall. However if the diet is poor, for example, lacking in protein and calcium, the body cells do not have sufficient nutrients for maximuim development of the growth potential.