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 4 TWIN STUDIES J. A, H. Waterhouse Twins make a dual demand on the attention of the clinical geneticist. In the first place there is their intrinsic interest as comparative rarities—as ill-understood genetic and physiological abnormalities in a species normally monotocous. Here the problem has been to devise adequate tests to assess the truth of many rather uncritical assertions. In the second place is that Galton, in 1857, was one of the first to recognize, the value of the History of Twins, as a Criterion of the relative powers of Nature and Nurture . The nature-nurture question, which, though he did not initiate, Galton assiduously pursued and sponsored for the rest of his life, was too often presented as a struggle of heredity versus environment rather than as a co-operative effort, and led to much bitter controversy and special pleading. Nevertheless, in the form in which it is generally used today the method of twin control can yield valuable information about the presence of a hereditary influence, though quantitatively inconclusive. The literature abounds with references to twins and twinning from all aspects and of all qualities. Comprehensive reviews may be found in Dahlberg (1926), Newman (1942), Gates (1946) and Gedda (1951). Modern work, not only on twins but particularly in the sphere of human genetics, has rendered much of the earUer work obsolete, or of historical interest only, and necessitated a reorien¬ tation of outlook. ORIGIN OF TWINS Monovular and binovular twins It is generally held that twins arise either from (1) the early division of the fertilized zygote into two separate, or partially separate, embryos, or (2) the fertilization of two ova released at about the same time. By reason of its mode of origin the former type of twin-pair—monozygous (MZ), monovular, or identical— must be of the same sex and possess exactly the same hereditary material in duplicate, whereas the latter type—dizygous (DZ), binovular, or fraternal—will be no more alike in sex or heredity than any pair of children of the same parents. And the existence of these two kinds of twins provides a natural basis for a controlled experiment to test the relative influences of heredity and environment. Dizygous twins, especially those of the same sex, are brought up in very much the same environment—an environment very much more similar than that of same-sex sibs of unequal ages—but differ in their heredity: whereas monozygous twins, separated very soon after birth and reared apart, provide the complementary material, genetically identical but diff'ering in their environment. Such a situation might represent the ideal for a controlled biological experiment but is very seldom realizable in practice, and in any case overlooks the period of intra-uterine life when the environments presumptively are very closely similar. Newman, Freeman 35