What is the only thing that should differ between experimental treatments

If I understand you correctly; the two experimental treatments are the experimental condition and the control condition.

The thing differing between them should be what you want to examine, and especially the thing you want to know of how it influences something. So for example if you want to know how alcohol influences something, the difference should be that one condition will include alcohol and the other won't.

Genetic engineering is the answer.

Ais the answer for you