A baby forms after a single egg is fertilized with the genetic information from both parents. Sometimes the egg splits up in two embryos that develop into twins throughout the course of the pregnancy. Any differences between twin offspring are attributed to mainly environmental influence and life experiences.
Recently a new breakthrough study showed that seemingly identical twins are not quite identical after all. Minor changes in their DNA can lead to major genetic divergence that develop as twins grow old. Scientists were able to discover this tendency by studying 19 pairs of identical twins and some of the diseases they were predisposed to develop in the course of time. In some pairs one of the siblings had missing genes on some chromosomes that meant a high risk of leukemia. One of the twins had indeed suffered from leukemia while the other one did not.
The genetic differentiation among identical twins could be more frequent than previously thought . The genetic code could be altered early in the fetal stages of development and it basically happens due to copy errors of the DNA along the process.