Master Race
The Master Race was created when the Saxon Master stepped into the Immortality Gate and transferred his genetic template to the 6,727,949,338 human inhabitants of the planet Earth on Christmas Day 2009.
Biology
As the Master Race, every human on Earth was rendered identical in every way to the Master with no apparent gender or age differences. It is unclear whether this temporarily turned the human race also into Time Lords. Also unclear is whether the change applied to infants and newborns, or whether, had the Master Race been allowed to continue, any possibility of procreation existed. Wilfred Mott wondered if dead bodies were affected; the Doctor's reaction suggested they were.
It was also never made clear whether they possessed the superhuman abilities that the Master had at the time due to his unstable form. However, they did appear to have the Master's feral hunger due to the latter's unstable form; as they indicated to Donna Noble that they were planning to devour her. Sarah Jane Smith's son Luke was also seemingly unaffected. (TV: The End of Time)
History
When the Saxon Master stepped into the Immortality Gate, an alien artefact reverse-engineered by billionaire Joshua Naismith, it sent out his genetic structure to nearly every human on Earth, turning everyone into exact copies of him. Several individuals escaped this transformation: Donna Noble, who appeared to have been protected by the influence of her DoctorDonna transformation; Wilfred Mott, who was protected by special radiation shielding at the time; the Tenth Doctor; and the alien scavengers Addams and Rossiter. Non-human entities such as Sarah Jane Smith's computer Mr Smith presumably recorded the events of the transformation. Any other non-human species present on Earth at the time (e.g. Mr Copper) would presumably likewise not have been affected. It also seems that Luke Smith may have been unaffected (perhaps due to having been artificially created by aliens) or, at the very least, somehow later learned of the events, as he referenced everyone having a "different face" when discussing Sarah Jane's attempt to cover up the event.
With all humans, including US President Barack Obama and the leaders of UNIT, now essentially the same man, the Master ordered his self-proclaimed Master Race to take control of the planet with the original Master as their undisputed leader. He planned to convert the planet into a warship. After the Doctor's escape, the planet's military resources were dedicated to searching for him.
Once the human race had all become the Master, they all heard the incessant drumming that had driven him mad over the years. Soon after, the Master realised he could use the combined mental power of the Master Race to send a signal of the drumming through time and space and help the Time Lords return to the universe. They made contact, and a White-Point Star diamond was sent to Earth. The Master Race recovered it and used it as a power source to pull Gallifrey out of the Time War to manifest near Earth, threatening the planet with destruction. Rassilon and other Time Lords appeared near the Immortality Gate.
When Rassilon reversed the Master's transfer of genetic code with a wave of his hand, the human race returned to their original form. (TV: The End of Time)
