Quantitative trait or character A phenotype where values for numerous individuals in a population are continuously distributed and that variation has both genetic and environmental causes. Value The phenotypic measurement of an individual in the units of trait measurement; the mean phenotypic measurement of a population.

Biologists have been aware of continuously distributed phenotypes since Mendel's time. After the recognition of Mendel's work in the early twentieth century there was a major controversy involving Mendelian genetics and quantitative genetics. The biometric school was a branch of genetics devoted to understanding the inheritance of continuously distributed phenotypes. Members of the biometric school pioneered the application of statistical methods to quantify and compare continuous phenotypic variation. Francis Galton (half-cousin of Charles Darwin) founded the biometric school through his study of human phenotypes and was an innovator in math and statistics as well as the founder of the eugenics movement (see Gillham 2001). Adherents of the biometric school argued that continuous traits were due to a distinct set of biological causes and could not be explained by Mendel's theory of particulate inheritance. Galton tried unsuccessfully to develop a model that explained the inheritance of quantitative traits without reference to Mendelian genetics (see Provine 1971; Bulmer 1998). In 1918, Ronald A. Fisher (the same R.A. Fisher who contributed the fundamental theorem of natural selection) published a seminal paper showing definitively how single Mendelian loci that individually produced discrete classes of phenotypes could combine to result in continuously distributed phenotypes (Fisher 1918).

The continuous distribution of phenotypes under Mendelian inheritance is due to polygenic variation. The continuous variation in phenotype results from the simultaneous segregation of several to many independent Mendelian loci. Figure 9.2 shows the phenotypic distribution for a trait determined by two independent Mendelian loci. In this two-locus illustration, the expected frequency distribution of phenotypes in the population is stepped and not smooth. However, the distribution clearly resembles

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