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twin
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Two twin boys with short curly hair, wearing matching navy blue shirts with yellow stripes, stand close together and smile outdoors. identical and fraternal twins
Learn about an Australian study on twins to investigate if the singing ability is inherited or environmentally determined
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Two twin boys with short curly hair, wearing matching navy blue shirts with yellow stripes, stand close together and smile outdoors.
twin Twin boys.

twin

biology
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twin, either of two young who are simultaneously born from one mother. Twinning, common in many animals, is of two biological kinds: the one-egg (monozygotic), or identical, type and the two-egg (dizygotic), or fraternal, type. The latter type is more usual and can be thought of simply as a litter of two. In humans, psychological studies of sets of identical twins, since they are genetically identical, have provided much otherwise unobtainable information on the relative effects of genetic endowment and environment. See also multiple birth.

This article was most recently revised and updated by Amy Tikkanen.