My story happens in fictional medieval age. There are the usual suspects of peasants, clergy, craftsman, merchants & aristocracy. There is also a specific class of hereditary servants. They work as domestic workers, and are above peasants and hired servants in class. Also they are more trusted, get better accommodations, more pay, and are usually better educated.
Hereditary servants are special since the aristocracy specifically bred them to look androgynous. They are normal humans but aristocracy selected the individuals with weakly pronounced secondary sex characteristics such hips, facial & body hair, Adam's apple, breasts etc. Males & females who didn't satisfy their specifications we removed from the genetic pool and were sterilized or had to change their class thus becoming normal peasants. Hereditary servants work only in households of aristocracy or very wealthy merchants.
Aristocracy wants to lock in androgynous traits, thus children of two servants need to look like a servant themselves. Thus children of two androgynous looking parents needs to be androgynous looking themselves.
Aristocracy has enough people to start with, they could use peasants which are something like serfs, or war captives as starting stock.
Aristocracy has knowledge of breeding animals, they've created different breeds of domestic animals such as dogs, horses, cows etc.
Aristocracy has no one to prevent them from choosing who will have children with whom, among the household servants. If baroness Meghan want to breed Jane and Tom, they could either acquiesce or try to run away from the country. Since household servants are chattel property, they don't have any say in that matter.
How many generations of selective breeding would they need to lock in androgynous traits, since the start of their endeavour?
The question is from the year they started creating the breed of servants, from naturally occurring stock. If there was no servants in year 800, only peasants who looked somewhat androgynous, how many generations would it take the aristocracy to lock in those traits.
I need something like rule of thumb for dogs which says about 7 generations https://www.quora.com/When-making-a-new-breed-how-many-generations-until-the-new-breed-is-considered-locked
In the beginning they just started with most androgynous peasants / war captives they could find. Afterwards they've used selective breeding to improve their servants. If the children were too masculine/feminine they were send to become common peasants, or simply removed from the pool.
Aristocracy keeps a registry of servants, thus you are purebreed if both your parents are purebreed. However servants are defined by their look. Even you are purebreed you would lose your class if you are abnormally masculine/feminine for a servant. Also you could be accepted even from a non servant parents if you fit the specifications.