Comment by Llamamoe

Genes code for production of protein. Protein have a specific, usually very narrow function. There's a lot of cases where a gene you want to add doesn't work right because of the difference between the organism it comes from and the one you're transplanting it to, but there isn't much of a way for it to have unpredictable results.

E.g. if you add a gene coding for an enzyme that turns molecule A into molecule B, most of the time there simply isn't a way for it to do anything, and the organism will usually adjust production of molecule A to compensate, so that there's barely even the tiniest difference other than it now producing molecule B.