The characters that Mendel studied are sometimes referred to as discrete characters because they can only be classified on an "either-or" basis (e.g., purple or white flowers, green or yellow seeds). Many characters cannot be classified in this manner because they vary in a population across a continuum (gradient). For example, the figure above illustrates that skin color in humans is a quantitative character. Quantitative characters usually indicate that the character is controlled by more than one gene (polygenic inheritance).
Figure 7 shows a simplification of the genetics of skin color in humans, with three genes interacting to determine the level of pigment in an individual's skin. The dominant alleles (A, B, and C) each contribute one "unit" of pigment to the individual, and their effects are cumulative such that individuals with more of these alleles will be darker than those with fewer alleles. The recessive alleles (a, b, and c) do not contribute any units of pigment. Therefore, skin color is related to the number of dominant alleles present in each individual's genotype.
A cross of two completely heterozygous parents produces seven phenotypes in their offspring, ranging from very light to very dark skin. The distribution of skin color in the offspring would resemble a bell-shaped curve because there would be more individuals with intermediate skin colors than either extreme. As the number of genes involved increases, the differences between the various genotypes become more subtle and the distribution fits the curve more closely. Other examples of polygenic inheritance in humans include height, hair color, and eye color. This helps to explain the variations in these characters that we see in different individuals.
Transcript for Complex Expression - Part II