One of the respondents in this thread suggested a 7-sex system with the following features:
An idea I came up with years ago was a variation on the male/female/carries-offspring idea. This species has three "base genders"; males, "egg layers" that are fertilized by males, and brooders who carry the eggs to term. To make things more complicated however any individual can have anything from just one to all three sets of genitals, resulting in seven possible genders.
The post was unclear about whether brooders contribute genetic material or just a womb/pouch/whatever to raise the young in. So, for the sake of argument, let's assume that the offspring receive genetic material only from the male and egglayer, and not the brooder. (However, combination brooder + another sex would probably primarily brood their biological offspring.)
My two questions are:
What would be a plausible genetic sex-determination system for this species?
Given your suggested system, what would a stable population distribution of the 7 sexes be?