Twins share a common intrauterine environment, but
though monozygous twins are genetically identical with respect
to their inherited nuclear DNA, dizygous twins are no more
alike than any other pair of siblings, sharing, on average, half
their genes. This provides the basis for studying twins to
determine the genetic contribution in various disorders, by
comparing the rates of concordance or discordance for a
particular trait between pairs of monozygous and dizygous
twins. The rate of concordance in monozygous twins is high for
disorders in which genetic predisposition plays a major part in
the aetiology of the disease. The phenotypic variability of
genetic traits can be studied in monozygous twins, and the
effect of a shared intrauterine environment may be studied in
dizygous twins.
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