A recessive gene was a type of gene that would not manifest unless two copies were present. For example, in Human-Skagaran hybrids the Skararan cranial genes were recessive, meaning that in an hybrid with only 1/4th Skagaran ancestry would look Human. (ENT: "North Star")

Within the Borg Collective, access to Unimatrix Zero was controlled by a recessive genetic mutation. (VOY: "Unimatrix Zero")

In "Favorite Son" The Doctor also talked about "recessive elements" in DNA, which may or may not have referred to recessive genes.

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