Twin Studies Are Helpful in Studying Polygenic Traits in Humans

For obvious reasons, heritability in humans cannot be calculated by making the environment constant (VE = 0) for a large number of individuals. However, one way to determine the relative contributions of genes and environment to polygenic traits in humans is to study genetically identical individuals produced by nature: identical twins. Identical twins are produced when a single sperm fertilizes a single egg, and the fertilized egg (called a zygote), rather than dividing to form one individual, early in its development splits to form two individuals. Since these embryos are the products of the same fertilized egg, they are genetically identical. However, twins also share a similar environment, except for those rare identical twins that have been raised apart. Therefore, in studies of twins, the percentages of twins that share the same trait, called percent concordance, are compared between identical (or monozygotic) twins and nonidentical (or fraternal or dizygotic) twins. The dizygotic twins share a similar environment but on average share only half their genes. By accounting for the similar environments and the proportion of shared genes, twin studies provide an estimate of heritability. Table 12.1 shows examples of traits studied in mono- and dizygotic twins. The data are expressed as percent concordance, that is percent of twins in which both twins had same or similar measure of that trait.

Some behavioral and other traits that are not strictly physical are also polygenic. For example, propensity for diseases, longevity, and intelligence are polygenic traits. A large twin study from Sweden tested over one hundred pairs each of identical and fraternal twins over the age of eighty. Percent concordance of some of their measurements of

Table 12.1 Percent Concordance Among Twins

Trait

Monozygotic

Dizygotic

Height

Weight

Birthweight

Childhood asthma

Coronary artery disease

Schizophrenia

93 92 67 65 46 60

64 63 58 40 12 10

cognitive ability is shown in figure 12.4. Intelligence, like other behavioral or mental traits, is a complex trait that can be measured in different ways. As shown in the graph, different measures of cognitive ability exhibit different degrees of genetic contribution. Thus mental speed has a much higher heritability than spatial ability. For complex behavioral and disease traits, twin studies provide an important tool to determine the genetic contribution to human traits.

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