Cogenitor
A cogenitor was one of the three genders of the Vissian species and was required for a male and female Vissian to successfully reproduce. The cogenitor did not pass on genetic material to the offspring they helped create; Dr. Phlox suggested that they may have supplied an enzyme during the  act which facilitated conception. 

Cogenitors made up only three percent of the Vissian population and their services for reproduction were consequently in high demand. However, in contrast to their rarity, cogenitors were not assigned a high position in Vissian society. Until at least the mid-22nd century they were relegated to a social status far inferior to that of the male and female genders. Cogenitors were not allowed to learn to read or write, were often dressed in the shabbiest of clothing, and had no input as to what couples they would help procreate or when. They were not even given a name. ()

Individuals 
 Charles

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Sex and reproduction