The study of twins / Peter Mittler.

  • Mittler, Peter J.
Date:
[1971]
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The study of twins / Peter Mittler. Source: Wellcome Collection.

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    168 Appendix В variance which is due to genetic differences between twins. As Huntley points out, this is not the same as estimating the heritability of a character¬ istic in the general population, because the genetic variance with¬ in pairs of DZ twins is only half that between unrelated people, while the environmental variance within pairs may be less, or the same, or even more than that between unrelated people (Huntley, 1969, personal communication). More recent heritability indices, such as that by Falconer (I960), aim to estimate the extent to which a characteristic is genetically determined in the general population. Falconer's argument is described by Huntley (1966) thus: For M Z twins all the additive genetic variance will be between- pairs because there are no unshared genes to cause differences within-pairs. . . . Differences within the pairs will be environ¬ mental differences. But if these environmental differences within pairs are less than the total environmental variance in the population, there will be a portion of this total variance which is common to members of the twin pairs and is an additional cause of similarity between them. MZ and DZ twins, by virtue of being two children of the same age growing up in the same family at the same time, may be assumed in general to be subject to much the same degree of environmental differences and similarities. Hence, in partitioning the variance for D Z twins, the within-pair environmental variance component and the between-pair common environmental vari¬ ance component will be the same as for MZ twins. However, DZ twins are on average genetically only half alike, so half the additive genetic variance will be within-pairs, and half between- pairs. . . . The differences between the between-pair components for MZ and DZ twins,which is the same as the difference between the covariances of the two kinds of twins, will be equal to half the genetic variance in the population ... (p. 237-8). Table 31, adapted from Falconer (1960), summarizes the above argument and shows how the components of the variance between and within pairs of twins may be par¬ titioned.