Clinical genetics / edited by Arnold Sorsby.

Date:
1953
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Clinical genetics / edited by Arnold Sorsby. Source: Wellcome Collection.

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    CHAPTER 6 POLYGENIC INHERITANCE Kenneth Mather Man is a difficult animal for genetical research. His families are small, his generations slow and his mating is uncontrolled—except, of course, for certain traditional prohibitions, such as the forbidden degrees of relationship in marriage in western civilization, which eliminate matings of types which are frequently among the most informative from the genetical point of view. Attempts have been made, notably by Galton and the biometrical school which followed him, to use man as a subject for primary genetical research, but these all failed. Our understanding of the nature, transmission and action of genes has come from experimental work on plants and other animals. Of these the most outstanding has been the fruit fly, Drosophila melanogaster, in which control of mating is easy, the generation time as short as two weeks at 25°C, famihes so large as often to run into hundreds or even thousands from a single pair, and culture so un- exacting that several hundreds of flies can be raised in a half-pint milk bottle. The rules of genetical behaviour have been gradually estabhshed by experiment with these more tractable species, and they have proved to show a remarkable similarity in all the great groups of the living kingdom, with the possible exception of bacteria which have yet to be tested adequately. Armed with this knowledge we can return to man, and even though we could not hope to elucidate the rules of genetics from his study, once knowing them we can show that he also conforms. Indeed, in view of the virtually universal validity of these basic principles, it would be very surprising if man did not behave in the same way. Even, however, the demonstration that man's genetical behaviour conforms to principles fully elucidated from other species is far from simple. We must generally infer the genetical constitution of parents from their offspring, and such inference must often be highly uncertain where the famihes are so small. To take an example, individuals showing the effects of a recessive gene will generally occur in the offspring of apparently normal parents. Having seen such an individual we can infer that the parents were both heterozygous for the gene. But only one child in four is expected to show the character. So with families of, say, two, an affected child will fail to appear in nine-sixteenths of cases. Thus we shall miss over half of the families of two which would be of interest to us. We shall obtain in fact a biased sample in-so-far as we must rely on ñnding an affected child in order to detect carrier parents; and in the famihes we actually find more than one- quarter of the children will show the affection. Only by the use of suitably elaborate mathematics and statistics can we show that the segregation does in fact conform to the mendelian expectation of f : | of all children from such parents. And with more complex phenomena like linkage the necessary mathe¬ matics become more elaborate still. 52