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How much of the difference in height among individuals is due to genetic differences, and how much is due to environmental differences? The slope of the line that relates the average phenotype of parents to the average phenotype of their offspring (in the case of Galton’s data, the blue line in Fig. 18.8) can answer this question because it provides a measure of the heritability of the trait. The heritability of a trait in a population of organisms is the proportion of the total variation in the trait that is due to genetic differences among individuals. For a complex trait, the heritability determines how closely the mean of the progeny resembles that of the parents. In Galton’s data, the slope of the line is 0.6 and therefore the heritability of height in this population is 60%.

The red dashed line in Fig. 18.8 presumes a hypothetical ideal trait in which variation is determined completely by genetic differences among individuals and the heritability is 100%. When heritability is 100%, the slope of the line representing the trait is 1. As we do with most complex traits, such as Nilsson-Ehle’s wheat seed color, we assume that each of a large number of genes contributing to the trait has two alleles, one that contributes to the trait and one that does not, and the genes undergo independent assortment. The number of alleles in the genotype contributing to the trait determines the phenotype in any individual for that trait. (The genetic model is like that shown in Fig. 18.4 but with many more genes.) In this ideal case in which heritability is 100%, the average phenotype of the offspring from any pair of parents will equal the average phenotype of the parents themselves. In the real world, the ideal is rarely encountered, and deviations from the red line result from complications such as dominance, genetic linkage, and epistasis (non-additive contributions of the alleles of different genes).

The black dashed line in Fig. 18.8 represents the opposite extreme, a hypothetical ideal trait in which variation is determined completely by differences in the environment among individuals and the heritability is 0%. When heritability is 0%, the line representing the trait has a slope of 0. As long as environmental effects are not transmitted from one generation to the next, the average phenotype of the offspring will be equal to the average of the population as a whole, no matter what the phenotypes of the parents. Although environmental effects are usually not transmitted in plant and animal populations, human populations are exceptional in showing cultural transmission of some environmental effects. For example, the average wealth of the offspring of rich parents is greater than that of the offspring of poor parents, and this difference is obviously due to transmission of the parents’ money, not their genes.

The term “heritability” is often misinterpreted. The problem is that, in non-scientific contexts, the word means “the capability of being inherited or being passed by inheritance.” This definition suggests that heritability has something to do with the inheritance of a trait. But “heritability” as used for complex traits means no such thing. It refers only to the variation in a trait among individuals, and specifically to the proportion of the variation among individuals in a population due to differences in genotype.

Hence, a heritability of 100% does not imply that the environment cannot affect a trait. As emphasized earlier, the environment is always important, just as oxygen is important to life. What a heritability of 100% means is that variation in the environment does not contribute to differences among individuals in a specific population. For example, if genetically different strains of chrysanthemums are grown in a greenhouse and subject to identical environmental conditions, then differences in flowering time have to be due to genetic differences, and the heritability of the trait would be 100%. Similarly, a heritability of 0% does not imply that genotype cannot affect the trait. A heritability of 0% merely means that differences in genotype do not contribute to the variation in the trait among individuals in a specific population. If genetically identical strains of chrysanthemums are grown in different environments, then differences in flowering time must be due to the environment, and heritability would be 0%.

Heritability therefore is not an intrinsic property of a trait. For chrysanthemum flowering time, the heritability in one case was 100% and in the second 0%. Heritability applies only to the trait in a particular population across the range of environments that exist at a specific time. Similarly, the heritability depicted by the slope of the blue line in Fig. 18.8 applies only to the population studied (205 pairs of British parents and their 930 adult offspring, in the late nineteenth century) and may be larger or smaller in different populations at different times. In particular, the magnitude of the heritability cannot specify how much of the difference in average phenotype between two populations is due to genotype and how much due to environment.

If the heritability of a trait can change depending on the population and the conditions being studied, why is it useful? Heritability is important in evolution, particularly in studies of artificial selection, a type of selective breeding in which only certain chosen individuals are allowed to reproduce (Chapter 21). Practiced over many generations, artificial selection can result in considerable changes in morphology or behavior or almost any trait that is selected. The large differences among breeds of pigeons and other domesticated animals prompted Charles Darwin to point to artificial selection as an example of what natural selection could achieve. Heritability is important because this quantity determines how rapidly a population can be changed by artificial selection. A trait with a high heritability responds rapidly to selection, whereas a trait with a low heritability responds slowly or not at all.

Quick Check 2 Many people are surprised to learn that, while each individual’s fingerprints are unique, the total number of fingerprint ridges is highly heritable, about 90% heritability in many populations. What does high heritability of this trait mean?