Identical twins. Twins that come from the same zygote and are, subsequently, the same genetically in terms of their nuclear DNA. Any differences between monozygotic twins later in life are mostly the result of environmental influences rather than genetic inheritance. Fraternal twins may look similar but are not genetically identical. Monozygotic twins may not share all of the same sequences of mitochondrial DNA. This is due to the fact that the mitochondria in a cell may have somewhat different versions of DNA, and the mitochondria can be dispersed unequally when a zygote fissions. Female monozygotic twins can also differ because of differences between them in X-chromosome inactivation. Subsequently, one female twin can have an X-linked condition such as muscular dystrophy and the other twin can be free of it.