A gene pool was the sum total of genetic information in a species.
The genetic pool of Nerva Beacon's human occupants was carefully balanced; the introduction of regressive traits was said to threaten their long-term survival. Lazar considered using his authority to destroy the Fourth Doctor and his companions lest they destabilise the gene pool. (TV: The Ark in Space)
The Dalek Emperor of the timeline in which Daleks underwent the Mutant Phase unintentionally allowed the introduction of wasp DNA into the Dalek gene pool which he had travelled back in time to prevent. (AUDIO: The Mutant Phase)
In the 26th century, Githians couldn't marry outside their species since they had trouble maintaining the purity of their gene pool. (AUDIO: Phobos)
Seneschal stole a tiny bit of the Seventh Doctor's skin, hoping he could use the constituent DNA to rebuild the gene pool of the Time Lords. (COMIC: The Sentinel!)
After Sontara was destroyed by the Rutan Host, the Sontarans took control of Pandora and made it their new homeworld. They used the racepool, the gene pool they brought with them, to recreate their race. (COMIC: Pureblood)