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.