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To see why this is so, let us consider the usual results of IQ studies on children who have been separated from their biological parents in infancy and reared by adoptive parents. Although these results vary quantitatively from study to study, they have three characteristics in common. First, because adoptive parents usually come from a better-educated population than do the biological parents who offer their children for adoption, they generally have higher IQ scores than those of the biological parents. Second, the adopted children have higher IQ scores than those of their biological parents. Third, the adopted children show a higher correlation of IQ scores with their biological parents than with their adoptive parents. The following table is a hypothetical data set that shows all these characteristics, in an idealized form, to illustrate these concepts. The scores given for parents are meant to be the average of the mother and father.

First, we can see that the scores of the children have a high correlation with those of their biological parents but a low correlation with those of their adoptive parents. In fact, in our hypothetical example, the correlation of children with biological parents is r 1.00, but with adoptive parents, it is r  0. (Remember that a correlation between two sets of numbers does not mean that the two sets are identical, but that, for each unit of increase in one set, there is a constant proportional increase in the other set—see the Statistical Appendix on statistical analysis at the end of this chapter.) This perfect correlation with biological parents and zero correlation with adoptive parents means that H2  1, given the arguments just developed. All the variation in IQ score among the children is explained by the variation in IQ scores among the biological parents, who have had no chance to influence the environments of their children. 

Second, however, we notice that the IQ score of each child is 20 points higher than that of its biological parents and that the mean IQ of the children is equal to the mean IQ of the adoptive parents. Thus, adoption has raised the average IQ of the children 20 points above the average IQ of their biological parents, and so, as a group, the children resemble their adoptive parents. So we have perfect heritability, yet high plasticity in response to environmental modification. An investigator who is seriously interested in knowing how genes might constrain or influence the course of development of any character in any organism must study directly the norms of reaction of the various genotypes in the population over the range of projected environments. No less detailed information will do. Summary measures such as H2 are not valuable in themselves. Knowledge of the broad heritability (H2) of a character 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 variance can itself be subdivided into two components to provide information about gene action and the possibility of shaping the genetic composition of a population. Our previous consideration of gene action suggests that the phenotypes of homozygotes and heterozygotes ought to have a simple relation. If one of the alleles encoded a less active gene product or one with no activity at all and if one unit of gene product were sufficient to allow the full physiological activity of the organism, then we would expect complete dominance of one allele over the other, as Mendel observed for flower color in peas. If on, On the other hand, physiological activity was proportional to the amount of active gene product, we would expect the heterozygote phenotype to be exactly intermediate between the homozygotes (show no dominance).

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