genetic engineering of animals

i feel that genetic engineering of animals is ok because that is how dogs and cats were brought into the world plus we may be able in the future to cure diseases by changing the genetic code of humans and in that case it would be a good thing to do but also there may be pultinal risks that may occur. yet we still dont know what the future holds but we have done this with animals and it turned out good so continueing it wont be that big of a deal. yes there are pros and cons to this but what is the worst that has happend yet?