LZMOOZYGOTIC & DIZYGOTIC TWINS1; Identical twins are also known as monozygotic twins. They result from the fertilization of a single egg that splits in two. Identical twins share all of their genes and are always of the same SEX. In contrast, fraternal, or dizygotic, twins result from the fertilization of two separate eggs during the same pregnancy.2; Monozygotic: Identical twins from the fertilization of one egg. Dizygotic: Fraternal twins from the fertilization of two eggs. 3; The classical twin STUDY design relies on studying twins raised in the same family environments. Monozygotic (identical) twins share all of their genes, while dizygotic (fraternal) twins share only about 50 PERCENT of them.4; Twins can be categorised as monozygotic or dizygotic depending on whether they stem from a single zygote (1 zygote = monozygotic) or are the result of 2 separate eggs fertilised by 2 separate sperm (2 zygotes = dizygotic). Dizygotic (non-identical) twins are MUCH more common than monozygotic (identical) twins.