Inbreeding, line breeding, out crossing
When you plan your breeding's, you will want to inbreed (the breeding together of close relatives such as sire/dam, son/daughter, brother/sister) only with great caution. Both good traits and bad can be exaggerated greatly.
Careful line breeding (the breeding together of more distant relatives) can be used to enhance and stabilize your best characteristics while helping you predict outcomes.
Out crossing (the breeding of two unrelated animals) will widen your gene pool-giving you more to work with, but continuous out crossing will just keep ;you guessing as to what results to expect.
Please pay very close attention to the third paragraph on out crossing because I believe this is one of the reason we are losing our dual purpose animal. You may buy a doe or buck that is both milk and meat but when you bring them home and breed to an unrelated mate this is where you begin to lose that dual purpose characteristic so much of the time. Buying good genetics to improve your herd is fine but if you continue to out cross you will soon lose those genetics. You have in a way just thrown you money away on that purchase.