One distinctive feature of the eye that varies from person to person is the number of creases on the eyelid. Eyelids that have no crease are called single eyelids, or monolids. Single eyelids are a genetic trait, but they are recessive, meaning these genes are expressed less often. On the other hand, double eyelid is the presence of a crease on the eyelid.
While genetics determine whether someone has monolids or double eyelids, surgery is available to help people born with monolids get double eyelids if they want.