Inbreeding increases homozygosity , helps to evolve a pureline in any animal, exposes harmful recessive genes that are eliminated by selection, helps in accumulation of superior genes and elimination of less desirable genes.
As there is selection at each step, the productivity of inbred population increases.
Continued inbreeding, especially close inbreeding –
reduces fertility & productivity called as inbreeding depression.
Whenever this becomes a problem, selected animals of the breeding population should be mated with unrelated superior animals of the same breed. This helps restore fertility and yield.