Twin Studies

Identical (monozygous) and fraternal (dizygous) twins provide a unique resource for studying the relative importance of heredity and environment. The technique of using twin study, originally devised by Francis Galton during the nineteenth century, has been widely used for the study of human inheritance.

Estimating Heritability from Twin Studies

Many years of research have been devoted to theorizing about ways to assess the importance of heredity in relation to environment, but no completely satisfactory method has been devised as yet. Following Fisher's exposition of a model for quantitating phenotypic variance among related individuals, Karl Holzinger proposed in 929 5 that differences between the phenotypic variance of fraternal twins (VDZ) and identical twins (VMZ), expressed as a fraction of the variance between fraternal twins, be taken as a decisive index of heritability (H) such that h = [Vdz - Vmz]/Vdz = [rMZ - r>z]/[ 1 - rDz] (5.2)

If the value computed for H derived from concordance rates is , the pheno-typic variation was assumed to be entirely attributable to heredity; and conversely, if H was 0, the phenotypic variation was assumed to be attributable to environment. This method can yield a deceptively reasonable estimate for heri-tability, but it has also been shown to give estimates that may be quite different from more soundly based estimates of heritability (see Table 5.2). 1 6 Although its use has long been used as a measure of heritability in twin studies, the Holzinger index is no longer deemed satisfactory for this purpose and it has been recommended that it be discontinued.17

In 1949, Lush defined heritability for quantitative characters in farm animal breeding studies as the fraction of the observed variance (VP) caused by differences in heredity (VG),18 such that heritability = VG/VP

Later, two types of heritability were distinguished: (1) ''broad sense heritability'' (hB2) (Equation 5.3) is the proportion of phenotype difference due to all sources of genetic variance (VP), regardless of whether the genes operate addi-tively or nonadditively; and (2) ''narrow sense heritability'' is the proportion of phenotypic variance due solely to additive genetic variance. Thus, if VG, VA, and VD refer to all genetic variance, to additive genetic variance, and to dominance effects, respectively, hB = Vg/Vp = (Va + Vd)/Vp (5.3)

Table 5.2 Heritability Estimates for Pharmacogenetic Twin Studies16

Drug

Measured parameter

rMZ°

f a rDZ

hB2 b

Hc (heritability)

Antipyrine

Plasma half-life

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