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Modern Genetic Analysis Chapter 18. Quantitative Genetics
The Heritability of a Trait

The most basic question to be asked about a quantitative trait is whether the observed variation in the character is influenced by genes at all. It is important to note that this is not the same as asking whether genes play any role in the character's development. Gene-mediated developmental processes lie at the base of every character, but variation from individual to individual is not necessarily the result of genetic variation. Thus, the possibility of speaking any language at all depends critically on the structures of the central nervous system as well as of the vocal cords, tongue, mouth, and ears, which depend in turn on the nature of the human genome. There is no environment in which cows will speak. But, although the particular language that is spoken by humans varies from nation to nation, that variation is totally nongenetic.

MESSAGE
The question of whether a trait is heritable is a question about the role that differences in genes play in the phenotypic differences between individuals or groups.

Familiality and Heritability

In principle, it is easy to determine whether any genetic variation influences the phenotypic variation among organisms for a particular trait. If genes are involved, then (on average) biological relatives should resemble one another more than unrelated individuals do. This resemblance would be seen as a positive correlation between parents and offspring or between siblings (offspring of the same parents). Parents who are larger than the average would have offspring who are larger than the average; the more seeds a plant produces, the more seeds its siblings would produce. Such correlations between relatives, however, are evidence for genetic variation only if the relatives do not share common environments more than nonrelatives do. It is absolutely fundamental to distinguish familiality from heritability. Traits are familial if members of the same family share them, for whatever reason. Traits are heritable only if the similarity arises from shared genotypes.

There are two general methods for establishing the heritability of a trait as distinct from its familial occurrence. The first depends on phenotypic similarity between relatives. For most of the history of genetics, this method has been the only one available; so nearly all the evidence about heritability for most traits in experimental organisms and in humans has been established by using this approach. The second method, using marker-gene segregation, depends on showing that genotypes carrying different alleles of marker genes also differ in their average phenotype for the quantitative character. If the marker genes (which have nothing to do with the character under study) are seen to vary in relation to the character, presumably they are linked to genes that do influence the character and its variation. Thus, heritability is demonstrated even if the actual genes causing the variation are not known. This method requires that the genome of the organism being studied have large numbers of detectable genetically variable marker loci spread throughout the genome. Such marker loci can be observed from electrophoretic studies of protein variation or, in vertebrates, from immunological studies of blood group genes. For example, within flocks, chickens of different blood groups show some difference in egg weight.

Since the introduction of molecular methods for the study of DNA sequence variation, very large numbers of variable nucleotide positions have been discovered in a great variety of organisms. This molecular variation includes both single nucleotide replacements and insertions and deletions of longer nucleotide sequences. These variations are usually detected by the gain or loss of sites of cleavage of restriction enzymes or by length variation of DNA sequences between two fixed restriction sites, both of which are a form of restriction fragment length polymorphisms (RFLPs). In tomatoes, for example, strains carrying different RFLP variants differ in fruit characteristics.

However, because so much of what is known or claimed about heritability still depends on phenotypic similarity between relatives, especially in human genetics, we will begin the examination of the problem of heritability by analyzing phenotypic similarity.

Phenotypic Similarity between Relatives

In experimental organisms, there is no problem in separating environmental from genetic similarities. The offspring of a cow producing milk at a high rate and the offspring of a cow producing milk at a low rate can be raised together in the same environment to see whether, despite the environmental similarity, each resembles its own parent. In natural populations, and especially in humans, this is difficult to do. Because of the nature of human societies, members of the same family not only share genes, but also have similar environments. Thus, the observation of simple familiality of a trait is genetically uninterpretable. In general, people who speak Hungarian have Hungarian-speaking parents and people who speak Japanese have Japanese-speaking parents. Yet the massive experience of immigration to North America has demonstrated that these linguistic differences, although familial, are nongenetic. The highest correlations between parents and offspring for any social traits in the United States are those for political party and religious sect, but they are not heritable. The distinction between familiality and heredity is not always so obvious. The Public Health Commission, which originally studied the vitamin-deficiency disease pellegra in the southern United States in 1910, came to the conclusion that it was genetic because it ran in families.

To determine whether a trait is heritable in human populations, we must use adoption studies to avoid the usual environmental similarity between biological relatives. The ideal experimental subjects are identical twins reared apart, because they are genetically identical but environmentally different. Such adoption studies must be so contrived that there is no correlation between the social environment of the adopting family and that of the biological family. These requirements are exceedingly difficult to meet; so, in practice, we know very little about whether human quantitative traits that are familial are also heritable. Skin color is clearly heritable, as is adult heightbut even for these traits we must be very careful. We know that skin color is affected by genes from studies of cross-racial adoptions and observations that the offspring of black African slaves were black even when they were born and reared in Canada. But are the differences in height between Japanese and Europeans affected by genes? The children of Japanese immigrants who are born and reared in North America are taller than their parents but shorter than the North American average, so we might conclude that there is some influence of genetic difference. However, second-generation Japanese Americans are even taller than their American-born parents. It appears that some environmental-cultural influence or perhaps a maternal effect is still felt in the first generation of births in North America. We cannot yet say whether genetic differences in height distinguish North Americans of, say, Japanese and Swedish ancestry.

Personality traits, temperament, and cognitive performance (including IQ scores), and a whole variety of behaviors such as alcoholism and of mental disorders such as schizophrenia, have been the subject of heritability studies in human populations. Many show familiality. There is indeed a positive correlation between the IQ scores of parents and the scores of their children (the correlation is about 0.5 in white American families), but the correlation does not distinguish familiality from heritability. To make that distinction requires that the environmental correlation between parents and children be broken, so adoption studies are common. Because it is difficult to randomize the environments, even in cases of adoption, evidence of heritability for human personality and behavior traits remains equivocal despite the very large number of studies that exist. Prejudices about the causes of human differences are widespread and deep, and, as a result, the canons of evidence adhered to in studies of the heritability of IQ, for example, have been much more lax than in studies of milk yield in cows.

Figure 18-8 summarizes the usual method for testing heritability in experimental organisms. Individuals from both extremes of the distribution are mated with their own kind, and the offspring are raised in a common controlled environment. If there is an average difference between the two offspring groups, the trait is heritable. Most morphological traits in Drosophila, for example, turn out to be heritablebut not all of them. If flies with right wings that are slightly longer than their left wings are mated together, their offspring have no greater tendency to be "right winged" than do the offspring of "left-winged" flies. As we shall see later, this method can also be used to obtain quantitative information about heritability.

MESSAGE
In experimental organisms, environmental similarity can often be readily distinguished from genetic similarity (or heritability). In humans, however, it is very difficult to determine whether a particular trait is heritable.

© 1999 by W. H. Freeman and Company. All rights reserved.