Qualitative, or discontinuous, characteristics possess only a few distinct phenotypes (Figure 17.1a); these characteristics are the types studied by Mendel (e.g., round and wrinkled peas) and have been the focus of our attention thus far. However, many characteristics vary continuously along a scale of measurement, with many overlapping phenotypes (Figure 17.1b). They are referred to as continuous characteristics, and they are also called quantitative characteristics because any individual’s phenotype must be described by a quantitative measurement. Examples of quantitative characteristics include height, weight, and blood pressure in humans, growth rate in mice, seed weight in plants, and milk production in cattle.

Quantitative characteristics arise from two phenomena. First, many are polygenic: they are influenced by genes at many loci. If many loci take part, many genotypes are possible, each producing a slightly different phenotype. Second, quantitative characteristics often arise when environmental factors affect the phenotype because environmental differences result in a single genotype producing a range of phenotypes. Most continuously varying characteristics are both polygenic and influenced by environmental factors, and these characteristics are said to be multifactorial.