Omegaverse. The rules differ from universe to universe, but here are my main rules:
We have two primary genders; male and female, and three secondary genders: alpha, beta and omega.
A person with a beta gender is no different from a normal person, while alphas and omegas produce pheromones designed to attract mates and express themselves. The smell of pheromones is unique to the individual, and often released when the alpha or omega is feeling emotional.
People with an omega secondary gender are able to get pregnant, no matter if they are male or female. People with an alpha secondary gender are able to impregnate, no matter if they are male or female.
Every few months an omega will go through their heat, while an alpha will go through their rut. During this time they will feel intense sexual desire, and release pheromones designed to attract either omegas or alphas. They can manage their ruts/heats by finding a partner to spend them with or using artificial suppressors, which is a tablet or needle designed to reduce the rut/heat 's effect on the body.
Omegas have a gland at the back of their neck that can be bitten by an alpha. If it is bitten, the alpha and omega will be mated for life, with the omega becoming dependant on the alpha's pheromones.