Another exception to Mendel’s rules is polygenic inheritance, which is when a trait is controlled by more than one gene. Often these traits are in fact controlled by many genes on many chromosomes. Each dominant allele has an additive effect, so the resulting offspring can have a variety of genotypes, from no dominant alleles to several dominant alleles. In humans, some examples of polygenic traits are height and skin color. People are neither short nor tall, as was seen with the pea plants studied by Mendel, which has only one gene that encodes for height. Instead, people have a range of heights determined by many genes. Similarly, people have a wide range of skin colors. Polygenic inheritance often results in a bell shaped curve when you analyze the population (Figure below). That means that most people are intermediate in the phenotype, such as average height, while very few people are at the extremes, such as very tall or very short.
Polygenic traits tend to result in a distribution that resembles a bell-shaped curve, with few at the extremes and most in the middle. There may be 4 or 6 or more alleles involved in the phenotype. At the left extreme, individuals are completely dominant for all alleles, and at the other extreme, individuals are completely recessive for all alleles. Individuals in the middle have various combinations of recessive and dominant alleles. Other polygenic traits in dairy cattle are of extreme economic importance in agriculture .
Most polygenetic traits are partially influenced by the environment. For example, height is partially influenced by nutrition in childhood. If a child is genetically programmed to be average height but does not get a proper diet, he or she may be below average in size.
Other examples of environmentally influenced traits are mental illnesses like schizophrenia and depression. A person may be genetically predisposed to have depression, so when that person's environment contributes major stresses like losing a job or losing a close relative, the person is more likely to become depressed.