You can actualy do both though it shouldn't be done too much. The plus point to doing this is to narrow your gene pool to known genetics or line breed a forum of morph ie. Bloodred corns. This can lead to problems as you also raise the chance of bringing out unwanted ressive genes ie, deformities and so on. So you'll want to out cross you lines every so often to keep them fresh. Breeding half siblings is also a good way of producing most morphs.