Look around you, look at your friends, and your family; look at your pets, and the dogs you see walking in the street; look at the insects crawling in the bushes outside, and the birds flying in the air.
How many of them are deformed? Not many, right? Maybe even none.
Yet, you carry in the region of five mutations, as do both your parents, all your children (if you have any) and all your friends. So does that tree you can see outside the window, the cat curled up in the sun, and the bird on the wing.
There's two reasons for this: the first is that the majority of mutations do not produce deformity, the second is that mutations that do produce deformity are usually fatal long before a creature reaches viability.
™ Version 4.2