Twin Sisters

The likelihood of a single fertilization resulting in identical twins is uniformly distributed in all populations around the world. This is in marked contrast to fraternal twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India and up to 24 in the US, which might mainly be due to IVF (in vitro fertilization). The exact cause for the splitting of a zygote or embryo is unknown.
In-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.
• Genetic and epigenetic similarity
Identical twins are genetically identical (unless there has been a mutation during development) and they are always the same sex. On rare occasions, identical twins may express different phenotypes (normally due to an environmental factor or the deactivation of different X chromosomes in female identical twins), and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly.