Heritability ($h^2$)

Heritability is the fraction of phenotypic variance in a specified biological population and environment attributed to genetic variation. It is a biological population parameter, not the genetic fraction of one individual's trait.

If observed variation is true-score variation $V_T$ plus independent measurement error $V_{ϵ}$, and genetic variance $V_G$ is part of $V_T$, then $h_{\mathrm{obs}}^2=V_G/(V_T+V_{ϵ})$. Writing measurement reliability as $q=V_T/(V_T+V_{ϵ})$ gives the displayed identity. Removing independent error can raise the heritability estimate without changing the genetic contribution to the true score. The result requires this measurement model; it does not identify measurement reliability by dividing arbitrary estimates from different longitudinal models.

The ratio of additive genetic variance to total phenotypic variance in a specified biological population and environment. It is not the fraction of one individual attributable to genes.

A twin study compares trait resemblance between twins with different degrees of genetic relatedness to separate genetic and environmental variance components.

The assumption that environmental similarity relevant to the measured outcome does not systematically account for the additional resemblance of monozygotic compared with dizygotic twins in the genetic model. It does not require identical treatment of all twins in every respect.

This is the single-zygote subtype of a twin.

Monozygotic twins develop from one zygote and are modeled as sharing essentially all additive genetic variation.

This is the two-zygote subtype of a twin.

Dizygotic twins develop from two zygotes and, absent assortative mating and other complications, share one half of additive genetic variation on average.

The ACE model decomposes trait variation into additive genetic effects $A$, common environmental effects $C$, and unique environmental effects $E$.

Falconer's formula estimates narrow-sense heritability from monozygotic and dizygotic twin correlations:It relies on equal relevant environments and a dizygotic additive-genetic correlation of one half.