Are identical twins truly identical?

Identical twins get dealt exactly the same DNA, but they may develop different traits down the lineóone may have allergies and the other may not, one may develop a particular disease and the other may not, one may be able to play the piano without ever learning how to read music while the other can't carry a tune with a dump truck. What accounts for these differences is something in their environmentópotentially as early as in uteroóaffected the expression of their genes differently. That something is called epigenetics.

While identical twins share the same genes, they're never truly identical because they may receive different amounts of non-genetic cellular material. In the womb, but even more so after birth and throughout life, identical twins continue to differentiate because they are exposed to different environments, a phenomenon known as "epigenetic drift."