Twins are two children produced in the same pregnancy and born in the same birth process. Fraternal (dizygotic) twins are born when two different eggs are fertilized by two different sperm. Identical (monozygotic) twins are born when a single egg is fertilized to form one zygote (thus the term monozygotic). The zygote later divides to produce two individual embryos which grow into two foetuses.
Approximately 1 in 350 births results in identical twins, and this rate is pretty much the same in the whole world. Fraternal twins are born with varying frequency depending on the country, from 6 per 1000 births in Japan to 15 per thousand births in India. In countries where IVF is commonly used, the proportion of fraternal twins is even higher, as more than embryo are commonly implanted to increase the chances of success of artificial fertilization.
Although both types of twins are born in the same birth process, there are fundamental differences between them.
Genetically, identical twins are just that – identical. They are, from the genetic point of view, exact copies of each other and they share 100%, or, technically, almost 100% of their DNA (almost 100% as it’s possible that mutations might happen after the zygote split). This is because identical twins originate from the same egg and the same sperm cell: they both get the same genetic information from the mother and from the father.
Fraternal twins are, genetically, as different as any other pair of full siblings are. It’s only by an accident of conception that they happen to share the uterus during the gestation. In some cases, fraternal twins, just like other siblings, are very similar to each other, in other cases they are very different. Genetically, fraternal twins share on average 50% of their DNA, just like non-twin full siblings, but this can vary between 0% and 100% (both 0% and 100% are rather unlikely).
Obviously, as fraternal twins largely share the uterine environment and are more likely to share their childhood environments and experiences, they might appear to be more similar than siblings that are not twins, but this is not necessarily due to greater genetic similarity.
Identical twins are *genetically* identical or almost identical, but it doesn’t mean they are exactly identical human beings. The expressions of the genes is influenced by the environmental factors (this is called epigenetic modification) which affects which genes are switched on and off, and how the genetic influence manifests itself. The environmental influences range from uterine environment to the experiences after the birth. For example, twins might be genetically predisposed to reach certain height, but the actual height will depend on nutrition in childhood and other factors (eg disease). Fingerprints of identical twins are different.
The greatest difference is observed between monozygotic twins that were separated at birth and reared apart, although often remarkable similarities can appear even between such twins.
Identical twins are almost always the same sex, except for very rare occasions of zygote with chromosomal abnormalities splitting unevenly. These are extremely rare occurrences, and normally identical twins will be of the same sex, while fraternal twins are as likely to be of different sexes as non-twin siblings.