Big & Small genes quite possibly, or perhaps Alpha Genes, though I don't remember that gene in that mod, so I don't 'think' it is. Maybe try Integrated Genes as well, but again I don't recall that gene being part of that mod. I do know the one you mean, I've seen Mr Samuel Streamer use it in his runs occasionally, and Big & Small is the only mod I can think of that might have it.

oh it is big and small genes! It's not listed on the steam page, but "what mod is that" says it's Big and Small. thank you! I just downloaded the 3 you listed just in case,