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)

Cauchemar encountered energy beings that had elevated themselves to that level of existence after discovering the gene for the soul. (PROSE: Vanishing Point)

The Dalek Factor was generally hidden amongst the genes. (PROSE: I Am a Dalek)

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)

7% of the genome of Alex Campbell, great-grandson of the Doctor, was made of Gallifreyan genes. (AUDIO: Relative Dimensions)