Most know how they are made, but for those who don't it's pretty simple. Daddy meets Mummy, and they love each other very much. Something involving a stork happens, and you get a single celled embryo called a zygote. If that zygote somehow splits into two embryos, you get two twins.
Conventional wisdom would have it that, since Daddy DNA and Mummy DNA had already gotten together by the time the embryo split, the two twins look so similar because they have the same mixture of DNA. It sounds reasonable. Even Wikipedia claims it to be true.
A recent article in the American Journal of Human Genetics (which appears to be open source - I can read it without being at work) looked at the Copy Number Variation of 19 pairs of monozygotic twins - those who should have identical DNA. Now epigenetic effects of monozygotic twins have been studied, and it is well known that there can be some startling differences between identical twins. However this is the first study which really looked at the actual information coded in the DNA of those twins.
What the paper presents is a curious bit of epigenetics (or possibly 'proper' genetics): monozygotic twins, who started off with exactly the same base DNA sequences and chromosomes, have different Copy Numbers.
Apart from the sheer interest value, this immediately places doubt on some of the very good work which has been previously done, on the basis that monozygotic twins have the same DNA, while dizygotic twins (or fraternal twins) have differing DNA (and thus any differences in genetic disease must have come about from those differences). Some of the CNV differences were in genes which are known to be connected to very serious diseases, like Parkinson’s disease and Lewy body dementia.
It all points to just how little we actually know about the pre-implantation phase of human development, and just how big an impact it can have on the future of health.