Gren was a sentient species of grey-skinned, short people. They are most noted for having three genders. In addition to Male and Female is the Trimale gender.
In the three genders the Male contributes the sperm, the female the egg, and the trimale carries and incubates the conceived child. Upon giving birth the trimale is reverted to a more primitive mind set and has to relearn many things. This led to in some Gren cultures to the trimales being treated as second class citizens.
The creation of the Gren dates back to the end of the "Great War" in the Sol Solar System. They were designed as workers and soldiers and the purpose of requiring a third gender came from an attempt to control their reproduction. The scientists were not able to remove the reproductive ability, but by creating a gender that required special catalysts to be present during a female's maturation in the womb they had hoped to keep their "product" under tight control. Everything became hopeless for the Gren's survival when the company that created them was dismantled and the chief scientists and leaders were put on trial for crimes against creation. However at some point the "Gren" began producing trimales without outside intervention and thus survived.
The genetic research to create such three gendered species was lost almost immediately after their creation. No one has been able to replicate it and any attempt to alter their code results in unstable offspring or losing the third gender.