O group

The O group was a very common human blood type. By late 2006, half of the human population of Earth had O blood. After the Sycorax hypnotised all of the A-positive population, Daniel Llewellyn, who had suggested a blood sample be placed on the Guinevere One space probe, apologized to Harriet Jones about the situation. While attempting to assuage Llewellyn's guilt, Harriet pointed out that half of the population would have been hypnotized had he used O group blood. (PROSE: The Christmas Invasion)