Such close breeding does not produce bad results when it is scientifically employed. The first thing is to establish that there are no outstanding weaknesses in the participating family. Such close breeding intensifies every family characteristic. Therefore, if a weakness exists, such as a tendency toward low milk production or some disease, this characteristic is intensified in the offspring. On the other hand, if there is a tendency for heavy milk production, long life, excellent pelts, greater size, disease resistance or, any other desirable quality, these characteristics will be intensified by inbreeding.
Line breeding is a form of inbreeding where offspring are bred back to a common ancestor.
By such means you again intensify the characteristics of the family, but at the same time there is less risk of intensifying some obscure or little understood undesirable characteristic which is present in the family.