Two hearts in a Human, the result of mutation.
Mutation was a process by which the genes of a lifeform were changed in some way by either internal or external stimuli. The lifeforms affected by this process were known as mutants. Mutation was a primary factor in evolution, as those mutations that increased an organism's overall likelihood of survival were usually incorporated into the genome of the lifeform's offspring. Other mutations were trivial, or detracted from an organism's viability for reproduction or continued existence.
After Phlox explained to Jonathan Archer that a dead Suliban which he had autopsied was atypical of his species, Archer asked if he was a mutant. Phlox concurred, but clarified that his modifications were not the result of accident but of genetic engineering. (ENT: "Broken Bow")
While observing Klingons in the 23rd century that lacked ridges on their foreheads, Dr. Julian Bashir asked Worf if they suffered from a genetic mutation. (DS9: "Trials and Tribble-ations")
Jack and other genetically-engineered Humans sometimes referred to themselves as "mutants". (DS9: "Statistical Probabilities", "Chrysalis")
In 2372, Julian Bashir suggested to Goran'Agar that his DNA structure underwent a random mutation which made him unable to be addicted to ketracel-white. (DS9: "Hippocratic Oath")
It was a mutation in Odo's body that led to him developing Section 31's morphogenic virus. (DS9: "The Dogs of War")
In 2374, the Srivani triggered mutations in the crew of the USS Voyager using genetic tags. (VOY: "Scientific Method")