Essentials of behaviour genetics / David A. Hay.

  • Hay, David A.
Date:
1985
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Essentials of behaviour genetics / David A. Hay. Source: Wellcome Collection.

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    individual variation, half the patients are of normal IQ and the rest range from mild to severe retardation. One reason for such variability may be the genetic background on which the disorder is imposed. Therefore we now consider the range of variation in the population as a whole. Polygenic inheritance Polygenic inheritance is such a fundamental part of behaviour genetics that Chapter 3 is devoted to explaining its principles. All that is being included here is sufficient to complete the account of mental retardation. As its name implies, polygenic inheritance involves the action of many genes on a particular trait. When we consider genes this way, we can no longer describe each individual gene in terms of its bio¬ chemical and other properties, but think only of each allele as adding or subtracting a given amount from the phenotype. Our approach is therefore quantitative rather than qualitative. The example of height will illustrate this distinction. People may be very short in stature because of such single gene effects as the autosomal dominant achondroplasia which reduces height by 40-50 cm. But the presence of such qualitative single gene effects is not enough to describe the range of heights in the normal population. Instead we must think of each person's height being determined by many genes acting quantitatively with some alleles increasing and some decreasing height. In addition, environmental effects such as nutrition will influence the final height. If intelligence and other behaviours are viewed in the same way, there is the possibility that some people may have an IQ of less than 70 and be classed as mentally retarded because of polygenic effects. They have no single-gene or chromosomal abnormality but score so low because of a combination of their environment and of many alleles for low intelligence. Figure 2.21 illustrates this hypothesis. iq Fig. 2.21 (a) The distribution of intelligence in the population as a whole and (b) the relative contributions of different causes of retardation in the group with IQs less than 70. (Modified from H. Zellweger and V. lonasescu (1978) in Medical Aspects of Mental Retardation (2nd edn) Carter C. H. (ed.), p.123. C. C. Thomas, Springfield, 111.) 68 CHAPTER 2
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