ABO - Alpha Beta Omega 
A world where Humans have a secondary biological gender besides M/F (usually determimed at school age). Omegas are the most vulnerable of the group, they have heat cycles and Omega Males can get pregnant. Alphas have superior genes and are at the top of the group. Alphas & Omegas have pheromones, they're a natural pair and have the Pair Bond thing while Betas are like normal people. Some stories have Dominant/Recessive variants, details differ depending on the Author~

Qianjun & Kunjun - are the Alpha & Omega in Ancient China

Tianqian' corresponds to 'Alpha', 'Zhongyong' to 'Beta', 'Kunze' to 'Omega', 'Xinxiang' to 'Pheromones', and 'Qingqi' to 'Heat cycles'.

Ancient A/B/O setting: A = Tianqian, B = Zedui, O = Dikun.