Regressive was a term used by the planners of the Ark to refer to humans with undesirable genetic traits. The genetic pool of the survivors was carefully balanced; the introduction of regressive traits was said to threaten their long-term survival. Amongst the volunteers for the project from Colony 9 a regressive faction existed, but were allowed because their regressive traits had a zero survival rate predicted across generations.
The Fourth Doctor and his companions were seen as regressive by Noah, due to their anachronistic speech patterns. Lazer considered using his authority to destroy them lest they infect the gene pool. (TV: The Ark in Space)