Identical twins are “monozygotic” which means they developed from a single fertilized egg that split. The likelihood of identical twins is about 3 in 1,000, almost the same Identical twins reared apart constitute two individuals with the same genotype (genetic constitution) reared in two different environments. The study of a series of such twins reared apart and reared together showed that influence of genetic and environmental factors on measured characteristics (phenotypes) of the twins is the least closely correlated.