Studies of identical twins, separated at birth, have shown that environment has more influence on the adult person each twin becomes than gentics do. However, some traits present in the adult twins could only be explained by the biology of identical twins (the fact that they are 2 individuals derived of a single egg) - traits like musical preference; which hand they wrote with; having identical hairstyles, etc.)
Below is a link that I have _just now_ researched that might give you more info.
