Why is inbreeding considered a harmful practice?

Explanation

Crossing of closely related animals within a breed for 4-6 generations is known as inbreeding. It involves mating of superior male and female animals to increase the homozygosity.
However, continuous inbreeding may bring the deleterious (harmful) recessive genes together (homozygous recessive condition) which are usually eliminated in selection. This may express the harmful effects of the deleterious genes. As a result of continued inbreeding, fertility and productivity of such animal is reduced. This condition is known as inbreeding depression.