Question: why do some twins end up identical, surely they have at least slightly different genes?

Kevin Arbuckle answered on 11 Mar 2014:

Identical twins come from the same ‘zygote’, which is the stage just after the mother’s egg is fertilised and so has both the mum and the dad’s genes. This means that identical twins (by definition) are genetically identical.

But remember that even identical twins won’t be exactly the same in every way, though often very similar, because our behaviour and even our body is partly from the environment rather than genes. By environment I mean everything that is around a person, including nutrition, experiences, and the mother when she is pregnant. These can all change the way that a person turns out, even with the same genes.

On the other hand, non-identical twins come from two different zygotes and so are no more similar than any normal brother and sister.