Genetics of Complex Traits
Some traits that are passed on from one generation to the next are controlled by more than one gene. In fact, genes on several different pairs of chromosomes can control the expression of a single trait. This means that there is great variation in the genotype of the trait, and there will also be a wide distribution of phenotypes.
An example involving traits controlled by more than one gene is skin color. For simplicity, this animation uses D to represent the alleles present in a person with very dark skin and L to represent the alleles in a very light-skinned person. In reality, many more genes control human skin color. As you can see, when a person with very dark skin color reproduces with a person with very light skin color, the resulting individuals will have medium colored skin. On the other hand, when two people with medium colored skin reproduce, the resulting children can have skin colors that range from very light to very dark.
The environment also affects the phenotypes of complex traits. This means that although two individuals have the same genotype they can have very different appearances. An example is human height. Identical twins should be exactly the same height, but if they are raised in different environments and given different foods, one may grow taller than the other. In the shorter of the two twins, the environment (poor nutrition) interacts with the genotype to change the phenotype.