0 a twin (= either of two children born to the same mother on the same occasion) who do not look exactly the same because they developed from two separate eggs: --
Ellen gave birth to two non-identical twin boys, Brian and Roger.
Identical twins, who have the same genes, were more likely to have a similar number of moles and freckles than non-identical twins.
Studies of identical and non-identical twins suggest that co-operative behaviour is heritable.
