So far, we’ve dealt primarily with characteristics that have only a few distinct phenotypes. In Mendel’s peas, for example, the seeds were either smooth or wrinkled, yellow or green; the coats of dogs were black, brown, or yellow; blood types were of four distinct types, A, B, AB, or O. Such characteristics, which have a few easily distinguished phenotypes, are called discontinuous characteristics.

However, many characteristics do not exhibit discontinuous phenotypes. Human height is an example of such a characteristic; people do not come in just a few distinct heights, but rather display a wide range of heights. Indeed, there are so many possible phenotypes of human height that we must use a measurement to describe a person’s height. Characteristics that exhibit a continuous distribution of phenotypes are termed continuous characteristics. Because such characteristics have many possible phenotypes and must be described in quantitative terms, continuous characteristics are also called quantitative characteristics.

Continuous characteristics frequently arise because genes at many loci interact to produce the phenotypes. When a single locus with two alleles encodes a characteristic, there are three genotypes possible: AA, Aa, and aa. With two loci, each with two alleles, there are 3² = 9 genotypes possible. The number of genotypes encoding a characteristic is 3n, where n equals the number of loci with two alleles that influence the characteristic. For example, when a characteristic is determined by eight loci, each with two alleles, there are 3⁸ = 6561 different genotypes possible for this characteristic. If each genotype produces a different phenotype, many phenotypes will be possible. The slight differences between the phenotypes will be indistinguishable, and the characteristic will appear continuous. Characteristics encoded by genes at many loci are called polygenic characteristics.

The converse of polygeny is pleiotropy, in which one gene affects multiple characteristics. Many genes exhibit pleiotropy. Phenylketonuria (PKU) is a genetic disease that results from a recessive allele; persons homozygous for this allele, if untreated, exhibit intellectual disability, blue eyes, and light skin color. The lethal allele that causes yellow coat color in mice is also pleiotropic. In addition to its lethality and its effect on coat color, the gene causes a diabetes-like condition, obesity, and an increased propensity to develop tumors.

Frequently, the phenotypes of continuous characteristics are also influenced by environmental factors. In this situation, each genotype is capable of producing a range of phenotypes, and the particular phenotype that results depends on both the genotype and the environmental conditions in which the genotype develops. For example, there may be only three genotypes at a single locus that encode a characteristic, but because each genotype produces a range of phenotypes associated with different environments, the phenotype of the characteristic exhibits a continuous distribution. Many continuous characteristics are both polygenic and influenced by environmental factors; such characteristics are called multifactorial characteristics because many factors help determine the phenotype.

The inheritance of continuous characteristics may appear to be complex, but the alleles at each locus follow Mendel’s principles and are inherited in the same way as alleles encoding simple, discontinuous characteristics. However, because many genes participate, because environmental factors influence the phenotype, and because the phenotypes do not sort out into a few distinct types, we cannot observe the distinct ratios that have allowed us to interpret the genetic basis of discontinuous characteristics. To analyze continuous characteristics, we must employ special statistical tools, which will be discussed in Chapter 17.