Line-breeding is the mating of sons to their dams, and of daughters to their sires. It can also be the breeding of male offspring to 'aunts' (sisters of their mother), and female offspring to 'uncles' (brothers of their sire). Line breeding is practiced widely in livestock breeding. It serves to fix the genotypes of the offspring...that is, the genetic make-up of the individuals to be used for breeding the next generation. It is important to know the genetic 'content' of an individual, particularly when it is not outwardly determinable. Line-breeding is sometimes, by observation of the next generation, the only way to determine the genotypes of the parents.
The phenotype of an individual is its observable properties. In-breeding, ie, breeding brother to sister, when the genotypes of the parent stock are known, is often used to attempt to fix the phenotypes of the following generation...that is, to fix certain visible or identifiable characteristics in a strain...such as colour, skin/coat type, conformation, or behaviour.
In-breeding will also cause any undesirable traits to be fixed in the next generation, so great care must be taken when choosing breeding stock, and culling may be necessary in the offspring for the overall good of the strain.