Monozygotic Twins (Identical Twins)
Monozygotic twins (also known as identical twins) occur when a single fertilized egg splits into two separate embryos which then implant into the uterus. Monozygotic twins share identical DNA at birth because they were originally the same single egg.
Identical twin DNA does change slightly throughout their lives because the environment can turn different genes on or off. Monozygotic twins are valuable research subjects because of their unique genetic circumstances. Twins who were raised in different households (due to adoption for example) often have similar traits, behaviors, habits, and likes. Sometimes they even drive the same vehicle and have the same type of job. This shows evidence of a genetic influence over these factors because the twins share the same genes but not the same environment.