A gene was a unit of heredity.
Vampires and Time Lords shared 98% of the same genes; regeneration was similar to vampiric abilities. (PROSE: Goth Opera) The V Factor was the gene responsible for turning someone into a vampire. (PROSE: Vampire Science)
Some Time Lords were capable of momentarily regenerating, or partially regenerating. Though this could use up a lot of regenerative energy, it would give the Time Lord a new set of genes, allowing them to fool genetic sensors. The Seventh Doctor used this method on the planet Purgatory to fool the genetic scanner used by the Imperial Landsknechte. (PROSE: Original Sin)
Professor Richard Lazarus' Genetic Manipulation Device accidentally unlocked an evolutionary dead-end lying dormant in human genes, turning him into a scorpion-like creature. (TV: The Lazarus Experiment)