Analysis of the validity of assumptions underlying a research on the heritability of quantitative traits.

Abstract

Heritability is a statistical measure, expressing the proportion of phenotypic variance that is explained by the genetic variance. Heritability of a given trait can vary in time and may differ between populations, which is inter alia associated with interactions and correlations between genotype and environment or population-specific differences. Therefore, heritability measures are modulated by both genetic and environmental factors. Twin studies constitute one of the methods used to determine contributions of genes and environment to a given trait. However, studies of this type suffer from some methodological limitations and require certain conditions to be satisfied; both these factors were analyzed in the present study. The studied material, including 1,263 pairs of male and female twins (among them 424 pairs of monozygotic twins and 839 pairs of dizygotic twins), born between the 22nd and 41st week of gestation, was collected at the Department of Perinatology and Gynecology, Poznan University of Medical Sciences, between 2002 and 2009. We analyzed the variance of six somatic traits. The contribution of genetic factors to the phenotypic variance of the studied twin population was estimated on the basis of the two heritability measures. Failure to meet the assumption of additivity of genetic and environmental effects probably is the cause of high values of heritability coefficients documented in our study. Analyzing the phenotypic variance of a trait, one cannot ignore the role of genotype-environment interactions and covariance, as these are their effects which increase the genetic variance and related coefficients of heritability. Therefore, it can be concluded that the genotypic component of the phenotypic variance of the trait is potentiated by the effects of various environmental factors. Our study confirmed that the critical remarks regarding the methodology of heritability estimation are fully justified. Consequently, the heritability indices of somatic traits should be considered only a provisional measure of genetic polymorphism, expressing an estimated relative contribution of genotypic variance to the phenotypic variance of a given trait.