Twin studies on anthropometrics: Exploring the role of genetic and environmental factors

Abstract

Twin studies using information on monozygotic (MZ) and dizygotic (DZ) twins have importantly increased our understanding of the effect of genetic and environmental factors on human growth and anthropometrics. Twin pregnancies differ in many ways from singleton pregnancies, most notably because of their earlier gestational age and lower birth weight. The differences between twins and singletons, however, largely disappear during the few first years of life, and thus in later life twins well represent the general population. Twin studies have shown that growth is under strong genetic regulation, and from 70 to 80% of the variation in adult height is explained by genetic differences between individuals. About half of the genes affecting height are the same during the growth period after 4 years of age, and genetic factors are also important when regulating the timing of pubertal growth. For body mass index the proportion of the trait variation explained by genetic differences is lower than for height, i.e., from 60 to 80%, and genetic correlations during the childhood and adolescence are lower than for growth in height. This is probably because body mass index is a more heterogeneous trait than height. Lower genetic variation for body mass index is found in physically active persons as compared to sedentary persons indicating that physical activity can suppress the function of genes predisposing to obesity. New molecular genetic methods have also recently improved opportunities in twin studies. An example is a study which found differences in gene expression in fat tissue within MZ pairs discordant for obesity. The nearly perfect natural experiment of twins will almost certainly create new opportunities in several areas of research in the post-genomic era.