Twins can be either monozygotic ("identical"), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ("fraternal"), meaning that they develop from two different eggs. In fraternal twins, each twin is fertilized by its own sperm cell.
Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous and random event. Identical twins are not dependent on race, country or ethnicity. The odds of having identical twins are the same for every couple, in every pregnancy, wherever they live in the world.
As yet, the reason for the occurrence of identical births is unknown. There is, therefore, something mysterious about the occurrence of identical twins. Monozygotic twinning occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide, that is about 0.3 percent of the world population, and is uniformly distributed in all populations around the world.
Identical Twins and Research
Identical twins spend their lives being compared for the benefit of science. They can assist psychologists in untangling the effects of nature versus nurture, or aid speech pathologists in understanding the causes of stuttering. As they share duplicate DNA, as well as the same upbringing, they are generally similar, if not exactly comparable, individuals.