And then the people with recessive genes die out and don't pass themselves on. That's the point, if there is such a thing, of DNA. In the long term, good genes win out over bad ones. If the available mating population is small, everyone has to mate a lot and throw away a few 'tards. That's that happens in pretty much all the species- young that don't keep up with their peers are either expelled from the nest or eaten by tigers. Look at the classic example of modern inbreeding- the European royals and their hemophilia. It popped up because of a random mutation, the weak ones died off, and the defect has now been bred out.
It is the same thing that happens with faulty dominant genes, just on a different time scale. If a dominant gene expresses itself in an environment where it isn't good, only 1/4 of the population survives (assuming it goes classical Mendellian [DD] [Dr] [rD] [rr])