In animal husbandry, if two closely related animals are mated for a few generations, it results in loss of fertility and vigour. Why is this so?

Explanation

It is an example of inbreeding. After two generations it leads to "inbreeding depression., characterised by loss of fertility and hybrid vigour. This happens because inbreeding results in homozygosity and recessive alleles tend to come together. As a result, recessive or harmful characters appear in the progeny.