Line breeding means repeatedly crossing related plants to fix a trait, and it works right up until vigour starts dropping off. Reduced germination, weaker growth and smaller yields are the warning signs that a line has been closed too long. Explanations of when to outcross and how breeders refresh a tired line are given on
https://breederregularseeds.com. Bringing in unrelated material costs you consistency for a generation, which is the trade nobody enjoys making.
