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Twin studies

Just over one in a hundred pregnancies produces twins. About a third of these are identical twins, with identical genes, making them genetic clones. Non-identical twins share around half their genes, so they are no more alike than ordinary brothers and sisters. In twin studies, scientists assume that both sorts of twins usually share the same environment: upbringing, diet and so on. But this is not necessarily the case.

Identical twins have identical genes.
Identical twins have identical genes.
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