Knowledge of heritability has great practical value because it allows us to predict the phenotypes of offspring on the basis of their parents’ phenotypes. It also provides useful information about how characteristics will respond to selection (see Section 17.4). In spite of its importance, heritability is frequently misunderstood. It does not provide information about an individual’s genes or the environmental factors that control the development of a characteristic, and it says nothing about the nature of differences between groups. This section outlines some limitations and common misconceptions of broad- and narrow-sense heritability.

HERITABILITY DOES NOT INDICATE THE DEGREE TO WHICH A CHARACTERISTIC IS GENETICALLY DETERMINED Heritability is the proportion of the phenotypic variance that is due to genetic variance; it says nothing about the degree to which genes determine a characteristic. Heritability indicates only the degree to which genes determine variation in a characteristic. The determination of a characteristic and the determination of variation in a characteristic are two very different things.

Consider polydactyly (the presence of extra digits) in rabbits, which can be caused either by environmental factors or by a dominant gene. Suppose that we have a group of rabbits all homozygous for a gene that produces the usual numbers of digits. None of the rabbits in this group carries a gene for polydactyly, but a few of the rabbits are polydactylous because of environmental factors. Broad-sense heritability for polydactyly in this group is zero because there is no genetic variation for polydactyly; all the variation is due to environmental factors. However, it would be incorrect for us to conclude that genes play no role in determining the number of digits in rabbits. Indeed, we know that there are specific alleles that can produce extra digits (although these alleles are not present in the group of rabbits under consideration). Heritability indicates nothing about whether genes control the development of a characteristic; it provides information only about causes of the variation in a characteristic within a defined group.

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AN INDIVIDUAL DOES NOT HAVE HERITABILITY Broad- and narrow-sense heritabilities are statistical values based on the genetic and phenotypic variances found in a group of individuals. Heritability cannot be calculated for an individual, and heritability has no meaning for a specific individual. Suppose that we calculate the narrow-sense heritability of adult body weight for the students in a biology class and obtain a value of 0.6. We could conclude that 60% of the variation in adult body weight among the students in this class is determined by additive genetic variance. We should not, however, conclude that 60% of any particular student’s body weight is due to additive genes.

THERE IS NO UNIVERSAL HERITABILITY FOR A CHARACTERISTIC The value of heritability for a characteristic is specific for a given population in a given environment. Recall that broad-sense heritability is genetic variance divided by phenotypic variance. Genetic variance depends on which alleles are present, which often differs between populations. In the example of polydactyly in rabbits, there were no alleles for polydactyly in our group of rabbits, so the heritability of the characteristic was zero. A different group of rabbits might contain many alleles for polydactyly, and the heritability of the characteristic might then be high.

Environmental differences can also affect heritability because V_P is composed of both genetic and environmental variance. When the environmental differences that affect a characteristic differ between two groups, the heritabilities for the two groups will often differ as well. Because heritability is specific to a defined population in a given environment, it is important not to extrapolate heritabilities from one population to another.

EVEN WHEN HERITABILITY IS HIGH, ENVIRONMENTAL FACTORS CAN INFLUENCE A CHARACTERISTIC High heritability does not mean that environmental factors cannot influence the expression of a characteristic. High heritability indicates only that the environmental variation to which the population is currently exposed is not responsible for variation in the characteristic. Let’s look at human height. In most developed countries, the heritability of human height is high, indicating that genetic differences are responsible for most of the variation in height. It would be wrong for us to conclude, however, that human height cannot be changed by the alteration of the environment. Indeed, in several European cities during World War II, height decreased owing to hunger and disease; furthermore, height can be increased dramatically by the administration of growth hormone to children. The absence of environmental variation in a characteristic does not mean that the characteristic will not respond to environmental change.

HERITABILITIES INDICATE NOTHING ABOUT DIFFERENCES AMONG POPULATIONS A common misconception about heritability is that it provides information about the nature of population differences in a characteristic. Heritability is specific for a given population in a given environment, so it cannot be used to draw conclusions about why populations differ in a characteristic.

Suppose that we measured heritability for human height in two groups. One group is from a small town in a developed country, where everyone consumes a high-protein diet. Because there is little variation in the environmental factors that affect human height in this group, and because there is some genetic variation, the heritability of height in this group is high. The second group comprises the inhabitants of a single village in a developing country. These people consume only 25% as much protein as those in the first group, so their average adult height is several centimeters less than that in the group from the developed country. Again, there is little variation in the environmental factors that determine height in this group because everyone in the village eats the same types of food and is exposed to the same diseases. Because there is little environmental variation and there is some genetic variation, the heritability of height in this group also is high. Thus, the heritability of height in both groups is high, and the average height in the two groups is considerably different. We might be tempted to conclude that the difference in height between the two groups is genetically based—that the people in the developed country are genetically taller than the people in the developing country. This conclusion is obviously wrong, however, because these differences in height are due largely to diet—an environmental factor. Heritability provides no information about the causes of differences between populations.

These limitations of heritability have often been ignored, particularly in arguments about the possible social implications of genetic differences between humans. For example, the results of a number of modern studies indicate that human intelligence, as measured by IQ (intelligence quotient) and other intelligence tests, has a moderately high heritability (usually from 0.4 to 0.8). On the basis of this observation, some people have argued that intelligence is innate and that enhanced educational opportunities cannot boost intelligence. This argument is based on the misconception that, when heritability is high, changing the environment will not alter the characteristic. In addition, because heritabilities of intelligence range from 0.4 to 0.8, a considerable amount of the variance in intelligence originates from environmental differences.

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Another argument based on a misconception about heritability is that ethnic differences in measures of intelligence are genetically based. Because the results of some genetic studies show that IQ has moderately high heritability, and because other studies find differences in average IQ among ethnic groups, some people have suggested that ethnic differences in IQ are genetically based. As in the example of the effects of diet on human height, heritability provides no information about causes of differences among groups; it indicates only the degree to which phenotypic variance within a single group is genetically based. High heritability for a characteristic does not mean that phenotypic differences among ethnic groups are genetic. We should also remember that separating genetic and environmental effects in humans is very difficult, so heritability estimates themselves may be unreliable. These limitations of heritability are summarized in Table 17.2. TRY PROBLEM 20