Twin Adoption Study

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Multifactorial traits are determined by many genetic and environmental factors acting together. They are also called complex traits or quantitative traits. Both the main effects and the interaction effects of genetic and environmental factors influence the traits. The causal pathways are not one-way drives or strait roads, genes can also influence environmental factors through behaviors. All the factors affecting multifactorial traits are causes and effects to each other. There are several approaches to unravel the components to complex traits or diseases. Two of them are twin study and the related twin adoption study. Twin studies leverage the advantages of different genetic makeup between dizygotic twins and monozygotic twins, and the environmental …show more content…
For qualitative traits, concordance rates can be calculated to compare the percentages of twin pairs in which both express the trait in MZ and DZ twins. If MZ twins have about twice the concordance rate of DZ, there might be genetic underpinning underlying the trait. For quantitative traits, correlations of traits among twins and heritability calculated from variances are used to define the genetic component of the trait. Heritability (h2) is calculated by dividing the difference between variance in DZ pairs and MZ pairs by the variance in DZ pairs. It varies from 0 to 1. A value higher than 0.7 or 0.8 suggests a strong genetic influence on trait. However, there are some limitations of this approach. They are: environmental exposures may not be the same even in MZ twins (equal shared environments assumption); MZ twins may have different gene expressions; effects of certain genotypes may be heterogeneous; ascertainment bias due to differential participation rate between affected and unaffected twins; zygosity determination problem; and generalizability …show more content…
MacGregor et al. (2000) suggests that studying the different associations of exposure and disease between discordant MZ and DZ twins enables the searching of environmental factors for traits. Also, knowledge on how genes and environment interactively influence traits gains by looking at the overall contribution of genes in the presence of environmental variation and gene expression in different environments. A way to break down a trait into genetic and environmental components is to use the components of variance model. The population-level variation (VP) of traits is separated into genetic (VG), shared environmental (VE), and unshared/random environmental (e) components. The genetic component can be further assumed to follow an additive (VA), dominance (VD), or epistasis model. Degree of relatedness among individuals determines the variance explained by each component. For example, when both additive and dominance models are assumed, the phenotypic covariance would be VA+ VD+ VE for MZ twins, but it would be 1/2VA+ 1/4VD+ VE for DZ twins. Because researchers hardly know the genetic models followed by in humans, a narrow sense heritability (VA/VP) which assumes only an additive model will be calculated for a trait in a population. By contrast, a broad sense heritability (VG/VP=(VA+ VD+ VE) /VP) can be calculated when factoring in each genetic
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