I'm making a world where small part of population are like Star Wars Force sensitives. They have high status in my world, as the sensitivity gives them certain advantages over ordinary people. A trained sensitive could defeat several Special Forces type of soldiers, though a dozen would probably kill him.
Sensitives are over-represented among the top of the society. To be born sensitive grants you a chance to become part of the elite. Having a sensitive among your family, friends & neighbors is an enormous prestige. It's the same as with aristocracy two century ago and movie/sport stars now.
A child born of two sensitives has a much larger chance of being a sensitive, than a child of two ordinary citizens, though sensitivity is an exception not a rule. Most children born to sensitive parents are not sensitive themselves, and sometimes a strong sensitive is born to normal parents.
Sensitivity is a weakly heritable trait.
The country runs an extensive system for finding and identifying sensitive individuals, regardless of their class. Every sensitive must be trained, whether they or their parents like it or not. Those who run away are considered fugitives, those who help them are considered traitors. On one hand an ordinary family could quickly rise in status for having sensitive child or even better several of them, on the other hand once preeminent families wither away for failing to do so.
The problem I face is that I want to limit the number of sensitives to a small part of the population, something like 1 in 10,000 would suffice.
The question is: How to reconcile small numbers of sensitives with sensitivity being a hereditary trait?
The ideas that don't work in my settings are below:
High mortality rate, due to wars & infighting. However female sensitives don't usually go to war, unless there is an emergency. Therefore they could simply repopulate the pool.
Caste system where noble bloodlines keep the right genes among themselves. The problem is that sensitivity is a badly needed trait for the society. The country wants to have more sensitives as they are very handy in wars with their neighbors. Being the ruling caste doesn't mean anything if your country is destroyed. The society doesn't care if your father is Michael Phelps or Joe Schmo, you could either swim fast or you don't. If you have potential you must join, it's not a destiny you could choose.
Reduced fertility. For women this might work, but the problem are men who could sire hundreds of children every year. Besides even among sensitive couples the chance to have a sensitive children is low. They might have one or none. This is an important part of my story, with preeminent families falling from grace, when they are unable to produce sensitive offspring. Similar to how a family could be devastated in ancient China when it was unable to have a member that passed imperial exam and become a government official.