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Modern Genetic Analysis Chapter 18. Quantitative Genetics
Determining Norms of Reaction

Remarkably little is known about the norms of reaction for any quantitative traits in any species. This is partly because it is difficult in most sexually reproducing species to replicate a genotype so that it can be tested in different environments. It is for this reason, for example, that we do not have a norm of reaction for any genotype for any human quantitative trait.

Three methods are available for replicating a genotype for norm of reaction studies. First, with plants such as Achillea, individual plants can be cloned by cutting them up and planting the pieces in different environments. Second, with domesticated species such as corn, inbred lines can be created by mating close relatives. By selfing (where possible) or by mating brother and sister repeatedly generation after generation, a segregating line (one that contains both homozygotes and heterozygotes at a locus) can be made homozygous. Third, in species such as Drosophila, in which the necessary dominant markers and crossover supressors are available, it is possible in a few generations of crosses to produce lines that are homozygous for one or two chromosomes of the entire chromosome set.

The purpose of creating homozygous lines is to produce groups of organisms within which all individuals are genetically identical. These homozygous lines can then be allowed to develop in different environments to produce a norm of reaction. Alternatively, two different homozygous lines can be crossed and the F1 offspring, all genetically identical with one another, can be characterized in different environments.

Ideally for a norm of reaction study, all the individuals should be absolutely identical genetically, but the process of inbreeding increases the homozygosity of the group slowly, generation after generation, depending on the closeness of the relatives that are mated. In corn, for example, a single individual is chosen and self-pollinated. Then, in the next generation, a single one of its offspring is chosen and self-pollinated. In the third generation, a single one of its offspring is chosen and self-pollinated, and so forth. Suppose that the original individual in the first generation is already a homozygote at some locus. Then all of its offspring from self-pollination will also be homozygous and identical at the locus. Future generations of self-pollination will simply preserve the homozygosity. If, on the other hand, the original individual is a heterozygote, then the selfing A / a × A / a will produce 1/4 A / A homozygotes and 1/4 a /a homozygotes. If a single offspring is chosen in this subsequent generation to propagate the line, then there is a 50 percent chance that it is now a homozygote. If, by bad luck, the chosen individual should still be a heterozygote, there is another 50 percent chance that the selected individual in the third generation is homozygous, and so forth. Of the ensemble of all heterozygous loci, then, after one generation of selfing, only 1/2 will still be heterozygous; after two generations, 1/4 after three, 1/8 In the nth generation

where Hetn is the proportion of heterozygous loci in the nth generation and Het0 is the proportion in the 0th generation. When selfing is not possible, brothersister mating will accomplish the same end, although more slowly.

Very few norm of reaction studies have been carried out for quantitative characters found in natural populations, but many more have been carried out for domesticated species such as corn, which can be self-pollinated, or strawberries, which can be clonally propagated. The outcomes of such studies resemble those given in Figure 18-6, which shows the norms of reaction for abdominal bristle number as a function of temperature for second chromosome homozygotes of Drosophila pseudoobscura. No genotype is consistently above or below the others. Instead, there are small differences between genotypes, and the direction of these differences is not consistent over a wide range of environments.

These factors have two important consequences. First, the selection of "superior" genotypes in domesticated animals and cultivated plants will result in very specifically adapted varieties that may not show their superior properties in other environments. To some extent, this problem is overcome by deliberately testing genotypes in a range of environments (for example, over several years and in several locations). It would be even better, however, if plant breeders could test their selections in a variety of controlled environments in which different environmental factors could be separately manipulated. The consequences of actual plant-breeding practices can be seen in Figure 18-7 on the following page, in which the yields of two varieties of corn are shown as a function of different farm environments. Variety 1 is an older variety of hybridcorn; variety 2 is a later "improved" hybrid. These performances are compared at a low planting density, which prevailed when variety 1 was developed, and at a high planting density characteristic of farming practice when hybrid 2 was selected. At the high density, the new variety is clearly superior to the old variety in all environments (Figure 18-7a). At the low density, however, the situation is quite different. First, note that the new variety is less sensitive to environment than the older hybrid, as evidenced by its flatter norm of reaction. Second, the new "improved" variety is actually poorer under the best farm conditions. Third, the yield improvement of the new variety is not apparent under the low densities characteristic of earlier agricultural practice.

The second consequence of the nature of reaction norms is that, even if it should turn out that there is genetic variation for various mental and emotional traits in the human species, which is by no means clear, this variation is unlikely to favor one genotype over another across a range of environments. We must beware of hypothetical norms of reaction for human cognitive traits that show one genotype unconditionally superior to another. Even putting aside all questions of moral and political judgment, there is simply no basis for describing different human genotypes as "better" or "worse" on any scale, unless the investigator is able to make a very exact specification of environment.

MESSAGE
Norm of reaction studies show only small differences between natural genotypes, and these differences are not consistent over a wide range of environments. Thus, "superior" genotypes in domesticated animals and cultivated plants may be superior only in certain environments. If it should turn out that humans exhibit genetic variation for various mental and emotional traits, this variation is unlikely to favor one genotype over another across a range of environments.

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