Drapetomania was a recognized mental illness before slavery was abolished. It was considered a disorder that made slaves want to be free. If we had had gene therapy back then no doubt scientists would've looked for the exact genes that correlate the most with wanting to be free as well as studied the neurochemistry and physiology. If we had technology on the level of gene therapy back then runaway slaves would be subjected to forcibly being cured of their 'disease'. They may have even been screed for potential factors and 'immunized' against 'drapetomania' just in case. And since slaves would all be 'treated' so they would be happy slaves slavery would never have ended.
Thinking about this should give you some pause. What will gene therapy be used for when it's more developed and widely available? What 'diseases' will it be used to treat? And what if we make a mistake about what to consider a disease? What if we 'cure' something that has hidden benefits or is being misjudged?
When you look throughout history many of the conquering nations slayed the opposing nations instead of enslaving them particularly because slaves are problematic and conscripts are not loyal. It is easier to do away with the entire problem then to feed, clothe, "screen", and manage a system of slaves, conscripts, concubines, and other forms of spoils from war.
If Gene Therapy were perfected prior to the end of slavery it would be used to enhance the farmers and the crops themselves not to ensure a hold for slaves. The entire purpose of slavery is to make up for the inabilities and insufficiencies of the human organism in the workforce.