Question

Twin/adoption studies often calculate moderately high heritability for behavioral traits, yet genetic association studies often fail to find candidate genes with large effects. How do you explain this apparent discrepancy?

Answer 1

Answer - Twin and adoption studies help us to  differentiate hereditary and environmental influences on human traits. Adoption help to determine if adopted children are more like their adopted  families, who contributed to the home environment, or to their biological parents, who contributed their genes.

Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: "identical" or monozygotic (MZ) twins share essentially 100% of their genes, which means that most differences between the twins such as in their height, susceptibility to boredom, intelligence, depression, are due to experiences that one twin has but not the other twin.Twins also share many aspects of their environment for e.g., uterine environment, parenting style, education, wealth, culture, community because they are born into the same family. The presence of a given genetic or phenotypic trait in only one member of a pair of twins called discordance provides a powerful way into environmental effects on such a trait.

Twins are also useful in showing the importance of the unique environment while studying trait presentation. Changes in the unique environment can stem from an event or occurrence that has only affected for e.g head injury or a birth defect that one twin has sustained while the other remains healthy.

The genes play an important role in the traits. By comparing many families with twins, we can understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.

Almost all traits are in part influenced by genetic differences, with some characteristics showing a stronger influence e.g. height, others an intermediate level e.g. personality traits and some complex heritabilities, with evidence for different genes affecting different aspects of the trait – as occurance of Austin.