Most cultures in the World of the Ledge recognize two genders: male and female, and the vast majority of individuals identify as men and women. But there is a growing realization that the gender drynamics of the planet are more complex, and cultures are only beginning to adapt. Why these peoples only appear sporadically or seemingly recently, no one can say for sure.
It is known that certain genders can only produce sons or daughters, while others are the product of three way unions, so for the sake of simplifying this discussion, we'll use the notation similar to the conventions of human chromosomes:
XX - Usually identify as female, very common.
XY - Usually identify as male, very common.
YY - Dead on arrival, not viable and extremely rare.
XXX - Usually identify as female, but with much more control over her own internal bodily functions. Is fertile her entire life, and generally able to handle multiple births without issue. Can reproduce asexually or with up to three partners (serving as a surrogate mother in this case). Generally larger, heavier-set, and wider-hipped than other women and most men. In the A Maze of Grain (Group 2) campaign, they'll be referred to as Mid-Wives.
XXY - Marginally more identify as female than male. Has more control over his/her external body, usually able to choose a moderately functional or appealing form with age (through directed growth). Able to bear children, but chance of pregnancy is usually reducted due to changes of sexual organs or bodily proportions.
XYY - Usually identify as male. Has moderate control over his body, usually more than XY men. Something of an alpha-male in most societies.
YYY - Naturally androgynous changelings. No strong leanings toward either gender, but usually able to assume multiple forms of both. Rare, but more common than most people realize. Not able to bear children, even in female form. Disproportionaly represented among new mortals and bastards.
See also: https://en.wikipedia.org/wiki/Telegony_(theory)