Mutation is the exactly right word, and it's evident in that the OP is having trouble with the TWINs.
All twins are genetic mutations. Think of identialcal twins. A sperm fertilizes the egg, the egg has a genetic mutation causing it to split in two. Boom twins caused by mutation. Now that is not to say the result is a MUTANT, as most twins I've met don't seem very much like the x-men.
Just pop a new seed.