A genetic engineer was a scientist or engineer that specialized in an expertise with the field of genetic engineering.
While Doctor Katherine Pulaski explained to Data what the word "eager beaver" meant, while viewing a virus, in 2365, she stated that she meant that, "in this case, "eager beaver" refers to some overachieving genetic engineer who – probably because of lack of anything better to do – has forced this strain of virus to mutate just so he can see how bad "bad" can get." (TNG: "The Child")