The study of twins / Peter Mittler.

  • Mittler, Peter J.
Date:
[1971]
Catalogue details

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

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

    167/196 (page 165)
    -167 Heritability estimates _ S (x¡ - g) (y¡ - a) _ S Xj У; - na^ i 2 2 * ns^ ns^ where Xiy¡ is the measurement on the /th pair of twins, n is the number of pairs is the variance of total sample about a a is the mean of all measurements. This correlation is interpreted as 'the total variance minus proportion of the total variance which arises from the fact that twin pairs differ from each other' (p. 113). The intra- class correlation is calculated by analysis of variance, and therefore yields an F-ratio which needs to be significant if the correlation is to be interpreted meaningfully. Heritability has been defined in the narrow sense by Falconer (1960) as 'the ratio of additive genetic variance to phenotypic variance', and refers to the population as a whole. The total phenotypic variance can be regarded as the sum total of variance due to genetic and to environmental factors. It can be partitioned into variance within and variance between pairs of twins. One of the earliest and most widely used ratios was Hol- zinger's Я. He proposed two alternative formulae, one based on within-pair variances only, and the other on the intraclass correlation : jrj varpz - varMz varoz JJ — ^MZ ~ ^DZ 1 ~ ''dz The second formula has been widely used, but has been criticized on several grounds, mainly because it is limited to a comparison of within-pair variances, and takes no account of between-pair variances. More precisely, it sub¬ tracts the non-genetic component, estimated from the MZ within-pair variance, from the within-pair DZ variance, and thus gives an estimate of the proportion of the total DZ