Is there a general rule about how long it can go on? Ie- to improve a certain trait a father rooster can mate with his daughters or sisters. If the practice is continued deformities are bound to happen...or are they? All of my silkies came from one place. I have whites, blacks, blues, and splashes. When I start to cull my flock, do I need to introduce new bloodlines or should I just keep going with what I have until I get one with one leg and two heads?