Most complex phenotypes are determined by multiple genes and the environment
Certain simple characters, such as those that Mendel studied in peas, differ in discrete, qualitative ways. Mendel used true-breeding parental pea plants that were either short or tall, had purple or white flowers, or had round or wrinkled seeds. But for most complex characters, such as height in humans, the phenotype varies more or less continuously over a range. Some people are short, others are tall, and many are in between the two extremes. Such variation within a population is called quantitative, or continuous, variation (Figure 12.14).
Figure 12.14 Quantitative Variation Quantitative variation is produced by the interaction of genes at multiple loci and the environment. These students (women [in white] are shorter; men [in blue] are taller) show continuous variation in height that is the result of interactions between many genes and the environment.
Sometimes this variation results largely from the alleles that an individual possesses. For instance, much of human eye color is the result of a number of genes controlling the synthesis and distribution of dark melanin pigment. Dark eyes have a lot of it, brown eyes less, and green, gray, and blue eyes even less. In the latter cases, the distribution of other pigments in the eye is what determines light reflection and color.
In most cases, however, quantitative variation is due to both genes and environment. Height in humans certainly falls into this category. If you look at families, you often see that parents and their offspring all tend to be tall or short. However, nutrition also plays a role in height: American 18-year-olds today are about 6 percent taller than their great-grandparents were at the same age. Three generations are not enough time for mutations that would exert such a dramatic effect to spread throughout the general population, so the height difference must be due to environmental factors.
Geneticists call the genes that together determine such complex characters quantitative trait loci. Identifying these loci is a major challenge, and an important one. For example, the amount of grain that a variety of rice produces in a growing season is determined by many interacting genetic factors. Crop plant breeders have worked hard to decipher these factors in order to breed higher-yielding rice strains. In a similar way, human characteristics such as disease susceptibility and behavior are caused in part by quantitative trait loci. Recently, one of the many genes involved with human height was identified. The gene, HMGA2, has an allele that apparently has the potential to add 4 millimeters to human height.