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Light changing genetic expression is environment changing genetic expression yes?

That.... is technically correct.

Huh, never thought of it like that.

the study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself.

That's the wikipedia definition of Epigenetics, and your right in that such a broad definition does include optogenetics in it.

Though I'm pretty sure some scientist are going to debate viciously for it being totally different Domains XS

But that's scientists for you... all wanting their domain to be more important XD