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Interaction of Heredity and Environment

All traits depend both on genetic and environmental factors. Heredity and environment interact to produce their effects. This means that the way genes act depends on the environment in which they act. In the same way, the effects of environment depend on the genes with which they work.

For example, people vary in height. Although height is highly heritable , environmental variables can have a large impact. For example, Japanese-Americans are on the average taller and heavier than their second cousins who grew up in Japan, reflecting the effect of environmental variables, especially dietary differences.

Phenylketonuria (PKU) is an excellent example of environmental modification of a genetically controlled effect. PKU is a form of mental retardation that results from toxic (~damaging) effects of abnormal breakdown of the essential amino acid, phenylalanine, which is found in all protein. The enzyme that breaks down phenylalanine is defective, so it accumulates and breaks down abnormally. So in PKU, a single gene can dramatically affect behavior: it is clearly a genetically influenced process.

But the effect of that defective gene expression depends on the environment in which it occurs. When this abnormal metabolism of phenylalanine was discovered, an effective treatment became available: a diet low in phenylalanine. The abnormal gene is still there, but because little phenylalanine is present in the diet, little toxic effects result. This defective gene was no longer expressed in mental retardation, because the environment was different: phenylalanine was greatly reduced in thefood the child with the defective gene ate.

The treatment for PKU illustrates the principle that the way a gene is expressed depends on the environment in which it is expressed. This principle is called interaction of heredity and environment This is a critically important idea. It says that all traits depend on the environment in which they are expressed, even traits that are very strongly affected by genetics. Conversely, all traits depend on the genotype on which they act, even traits that are very strongly affected by environmental differences.

Another illustration of this idea is the effect of environments on genetically "maze bright" and "maze dull" rats (see Exercise asgn1p). These strains were selectively bred to learn a maze quickly or slowly. They turn out to differ in maze learning only if they grow up in a "normal" laboratory environment. If they grow up in a very restricted environment, both kinds learn equally slowly. If they grow up in an enriched perceptual environment (a rat Disneyland), both kinds learn equally quickly (Cooper & Zubek. 1958)

A film (Friedman, 1958) shows the effect of indulgence and discipline on puppies from four breeds of dogs. Half of two litters from four breeds (basinji, beagle, sheltie, terrier) are "indulged" twice daily; the other half are trained on obedience tasks. The different treatment had little effect on the basenjis and terriers but a lot on the beagles and shelties. Some of the differences were in the opposite direction for these breeds. For example, adult indulged beagles became rather wary of humans whereas the disciplined beagles acted normally. In contrast, the indulged shelties acted normally, whereas the disciplined ones were very quiet and unresponsive. For a more complete summary of that film, click HERE.