Heritability

An often repeated claim of those who analyze the social status of blacks and whites is that differences between the groups in average IQ test scores is a result of genes that directly cause that difference in mental ability. This claim is said to be validated by observations that point to the heritability of IQ test performance. For example, the IQ scores of adopted children are correlated with the IQ scores of their biological parents, even though the children were adopted at an early age. That is, the higher the IQ scores of the biological parents, the higher the IQ scores of their children raised by adopting parents, indicating an effect of heredity. What this observation, however, does not take into account is that the average IQ score of the adopted children as a group is much higher than the average score of the biological parents and is equal to the average IQ score of the adopting parents as a group. That is, being raised by the adopting parents, who, in fact, have higher IQ scores as a group than do their biological parents, results in an increase in the IQ scores of the children over those of their biological parents.

Differences between individual organisms in measurable characteristics such as weight, growth rate, susceptibility to physical disorders, or behavior are a consequence of three interacting causes: genetic differences, environmental differences, and random developmental events. In the absence of detailed experimental modifications of the developmental process by controlled genetic and environmental manipulations, it is impossible to provide an accurate description of the causal pathways leading to a mature organism. Plant and animal breeders, however, need to choose breeding stocks and techniques of artificial selection that enable them to produce, as quickly and efficiently as possible, higher-yielding and more disease-resistant agricultural varieties. For this purpose, they developed, in the first half of the twentieth century, techniques for estimating the “heritability” of observed differences. The heritability of a trait in a particular variety estimates what proportion of the difference between the measurement of the trait in a population and the measurement of it in a specially selected group from that population would be preserved in the next generation if only the selected group were used as parents for that next generation. If the selected group of parents is three inches taller than the average of the population, how much taller than average will their offspring be? The proportion of selection difference that appears in the next generation is the realized heritability of the trait. If the next generation is only 1.5 inches taller, on average, then the realized heritability is 50 percent.

Such a measure is only useful for breeding experiments if the environment is kept the same in the two generations. If the environment is not the same, it cannot be known whether the selection really worked or whether it was the result of an environmental improvement.

A more sophisticated experimental approach to this same problem is to vary the environment and the genetic parentage of the organisms in a controlled way and to then analyze the variation in the offspring to estimate what proportion of that variation can be attributed to genetic differences, what proportion to environmental differences, and how much specific interaction there is between the genetic and environmental variations. A common technique is observing the amount of similarity between relatives of various degrees while keeping the distribution of environments the same for all of the relatives. Such an analysis, however, is not an analysis of causal pathways, and it is a serious error to confuse such an analysis of variation with an analysis of causation (Lewontin 1976). The same set of genetic lines, when tested in a different average environment but with the same amount of environmental variation, will give a different estimate of heritability, and the lines may be in a different relative order in their performance.

Despite this meaning of heritability, human geneticists and psychologists have repeatedly estimated heritability of human traits, especially traits of mental performance, in the process making serious methodological and conceptual errors. First, because they cannot control human developmental environments, many studies have either ignored the problem or made a variety of convenient but untestable assumptions about environmental similarities. Identical twins raised apart, for example, may be separated at various times after birth or may be raised by close relatives in the same locality. Second, interpretation may confuse the analysis of the variation with a separation of genetic and environmental causes, arguing, for example, that because the estimate of heritability of IQ was 70 percent, then only 30 percent of the observed differences in IQ could be eliminated by environmental interventions.

Finally, there is a confusion of the heritability of a trait within a population and the heritability of differences between populations. This is important in analyzing claims about differences between races. Differences within a population can be entirely genetic, while differences between populations can be entirely environmental. As an example, suppose a handful of seed from a genetically variable population of maize is planted in a chemically controlled, uniform environment. All the differences in growth among individuals will then be the consequence of their genetic differences. Suppose a second sample of seed from the same variety is grown in a uniform environment like the first, except it is deficient in an important nutrient, leading to all the seeds growing poorly. Again, the variation among plants within that environment will be entirely genetic, but the difference between the two groups of seed will be entirely environmental. In like manner, the differences in mental performance among individual children within a racial group may be strongly influenced by genetic differences, yet the differences between the groups may be the result of the different social and educational environments in which the groups find themselves.

SEE ALSO Genetic Variation Among Populations; Genetics, History of; IQ and Testing.

BIBLIOGRAPHY

Bodmer, Walter F., and Luigi L. Cavalli-Sforza. 1970. “Intelligence and Race” Scientific American 223 (October): 19–29.

Lewontin, Richard C. 1974. “Annotation: The Analysis of Variance and the Analysis of Causes.” American Journal of Human Genetics 26 (3): 401–411.

———, Steven Rose, and Leon J. Kamin. 1984. Not in Our Genes. New York: Pantheon.

R. C. Lewontin

heritability A measure of the degree to which a phenotype is genetically influenced and can be modified by selection. It is represented by the symbol h²: this equals V_a/V_p where V_a is the variance due to genes with additive effects (known as the additive genetic variance) and V_p is the phenotypic variance. The variance V may also be written as s². Parent–offspring correlations are estimates of familiality and not of heritability: they cannot account for environmental correlations between relatives. This definition of heritability is a narrow one: heritability in the broad sense (represented by H²) is the fraction of total phenotypic variance that remains after exclusion of the variance resulting from environmental effects. Estimates of heritability are used widely by plant breeders to predict the likely effects of selection. If heritability estimates are low for a particular character, this indicates that the character is mainly influenced by the environment and suggests that the response to selection would not be rapid.

heritability A measure of the degree to which the variance of a particular phenotype is caused by genetic factors. It is given by a value between 0 and 1 and effectively measures the extent to which offspring resemble their parents relative to the population mean. Estimates of heritability are important in applied genetics, especially in agriculture and horticulture, because they enable prediction of the response of a population to artificial selection. The higher the heritability value, the greater the response, although heritability declines after several generations of artificial selection due to increasing homozygosity. The term is used in two different ways. Heritability in the narrow sense is the proportion of phenotypic variance due only to the additive genetic effect of all the polygenes controlling a particular trait; hence it measures the proportion that is transmissible to offspring and therefore amenable to selection. Heritability in the broad sense considers other genetic but nontransmissible factors as well, including dominance and epistasis; it is used, for example, in psychology to quantify genetic and environmental influences.

heritability The measure of the degree to which a phenotype is genetically influenced and can be modified by selection. It is represented by the symbol h²: this equals V_a/V_p where V_a is the variance due to genes with additive effects (known as the additive genetic variance) and V_p is the phenotypic variance. The variance V may also be written as s². Parent-offspring correlations, as used in studies of the heritability of human IQ, are estimates of familiality and not of heritability: they cannot account for environmental correlations between relatives. This definition of heritability is a narrow one: heritability in the broad sense (represented by H²) is the fraction of total phenotypic variance that remains after exclusion of the variance due to environmental effects.

heritability A measure of the degree to which a phenotype is genetically influenced and can be modified by selection. It is represented by the symbol h², which is equal to the additive genetic variance divided by the phenotypic variance. Parent–offspring correlations are estimates of familiality and not of heritability: they cannot account for environmental correlations between relatives. This definition of heritability is a narrow one: heritability in the broad sense (represented by H²) is the fraction of total phenotypic variance that remains after exclusion of the variance due to environmental effects. Estimates of heritability are used widely by plant breeders to predict the likely effects of selection. If heritability estimates are low for a particular character, this indicates that the character is mainly influenced by the environment and suggests that the response to selection would not be rapid. See also Fisher's fundamental theorem.