how common is identical twins
Insights From Identical Twins. Because identical twins develop from a single fertilized egg, they have the same genome. So any differences between twins are due to their environments, not genetics. Recent studies have shown that many environmentally induced differences are reflected in the epigenome.
However, the fact that both identical twins in a pair don't develop the disease 100% of the time indicates that other factors are involved. Identical twins (left) share all their genes and their home environment. Fraternal twins (right) also share their home environment, but only half of their genes.