Everything Else . As mothers of identical twins would tell you, there are always subtle and even not so subtle physical differences between them, and this is true in the case of their fingerprints as well.
Identical twins result when a single fertilized egg divides into two replicas of itself.
Thus, each side has the same combination of genes as the other. However, identical genes almost never produce identical results because of what scientists call penetrance. Penetrance is a measure of the frequency with which any given gene fully expresses its trait. Almost no genes have 100 percent penetrance. Sometimes, a gene will be expressed in one identical twin but to a lesser degree or not at all in the other.