A study of twins for dominant handedness (right/left) had the following % concordance values: monozygotic twins were 67%, dizygotic twins were 74%. What is the correct interpretation for handedness in this data set?
The data means that the monozygotic twins have less chance to have same dominant handedness than the dizygotic twins. This represent the fact that, since monozygotic twins have more genetic similarity, so, dominant handedness is not entirely a genetic trait.
Moreover, the monozygotic twins have more probability than dizygotic twins to have different dominant handedness. This might be due to reason delayed embryo splitting of monozygotic is related with mirror-imaging effects when the division happens after the formation of an axis of bilateral symmetry. So in situation like this , opposite handedness in the same monozygotic pair of twins is expected. It is thought that this relation between handedness in twins mirror imaging is because monozygotic twin share the same placental blood supply, and is surrounded by the same chorionic membrane.
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