1. Heritability of a trait

1. 1. 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. • For example, 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.

2. 2. 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 that a plant produces, the more seeds that 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.

3. 3. 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 different countries has demonstrated that these linguistic differences, although familial, are nongenetic. • The highest correlations between parents and offspring for any social traits are often those for political party and religious sect, but they are not heritable. However, the distinction between familiality and heredity is not always so obvious. • For example, The Public Health Commission, which originally studied the vitamin deficiency disease pellagra in the southern United States in 1910, came to the conclusion that it was genetic because it ran in families.

4. 4. Heritability of behavioral traits • Personality traits, temperament, and cognitive performance (including IQ scores), as well as 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.

5. 5. Quantifying Heritability • If a trait is shown to have some heritability in a population, then it is possible to quantify the degree of heritability. We have seen that the variation between phenotypes in a population arises from two sources. • First, there are average differences between the genotypes; second, each genotype exhibits phenotypic variance because of environmental variation. The total phenotypic variance of the population VP can then be broken into two parts: the variance between genotypic means VG and the remaining variance VE. The former is called the genetic variance, and the latter is called the environmental variance • The degree of heritability can be defined as the part of the total variance that is due to genetic variance: H2 = VG / VP=VG/(VG + VE ) • H2, so defined, is called the broad heritability of the character.

6. 6. The Meaning of H2 • Attention to the problems of estimating broad heritability distracts from the deeper questions about the meaning of the ratio when it can be estimated. Despite its widespread use as a measure of how “important” genes are in influencing a trait, H2 actually has a special and limited meaning. • There are two conclusions that can be drawn from a properly designed heritability study. • First, if there is a nonzero heritability, then, in the population measured and in the environments in which the organisms have developed, genetic differences have influenced the variation between individuals, so genetic differences do matter to the trait. This is not a trivial finding and is a first step in a more detailed investigation of the role of genes. • Second, the value of the H2 provides a limited prediction of the effect of environmental modification under particular circumstances. If all the relevant environmental variation is eliminated and the new constant environment is the same as the mean environment in the original population, then H2 estimates how much phenotypic variation will still be present. So, if the heritability of performance on an IQ test were, say, 0.4, then, if all children had the same developmental and social environment as the “average child,” about 60 percent of the variation in IQ test performance would disappear and 40 percent would remain.

7. 7. Additive and Dominance Variance • Knowledge of the broad heritability (H2) of a trait in a population is not very useful in itself, but a finer subdivision of phenotypic variance can provide important information for plant and animal breeders. The genetic variation and the environmental variation can themselves each be further subdivided to provide information about gene action and the possibility of shaping the genetic composition of a population. • When a heterozygote A/a is not exactly intermediate in phenotype between the two homozygotes A/A and a/a, then all the genetic variation for phenotype cannot be accounted for by differences in the average effect in the population of A and a alleles. Some of the genetic variance is the result of the deviation of the heterozygote from the phenotype expected from a simple averaging of the two allelic effects. The total genetic variance in the population can then be subdivided into additive genetic variation, VA the variance that arises because there is an average difference between the carriers of a alleles and the carriers of A alleles, and a component called the dominance variance VD, which results from the fact that heterozygotes are not exactly intermediate between the monozygotes. Thus VG = VA + VD

8. 8. Heritability in the narrow sense • Thus, we define a new kind of heritability, the heritability in the narrow sense (h2), as h2 = VA / VP=VA/(VA + VD + VE ) • It is this heritability, that is useful in determining whether a program of selective breeding will succeed in changing the population. The greater the h2 is, the greater the difference is between selected parents and the population as a whole, which will be preserved in the offspring of the selected parents. • Even though h2 is a number that applies only to a particular population and a given set of environments, it is still of great practical importance to breeders. • A poultry geneticist interested in increasing, say, growth rate is not concerned with the genetic variance over all possible flocks and all environmental distributions. Given two possible flocks, the question becomes: can a selection scheme be devised to increase growth rate and, if so, how fast? If one flock has a lot of genetic variance and another only a little, the breeder will choose the former to carry out selection. If the heritability in the chosen flock is very high, then the mean of the population will respond quickly to the selection imposed, because most of the superiority of the selected parents will appear in the offspring. The higher h2 is, the higher the parent–offspring correlation is. If, on the other hand, h2 is low, then only a small fraction of the increased growth rate of the selected parents will appear in the next generation.