Definition of Inbreeding Depression
Inbreeding depression is a phenomenon in which the organisms that are closely related to each other through ancestral basis, are allowed to mate. But when they produce homozygotes, they are not fit for the population, and hence, the reproductive success of a population is decreased. When a population is small, inbreeding is quite common and the deleterious alleles are easily passed on.
But this lowers the chances of survival and reproduction in the next generation. On the contrary, outbreeding has many positive aspects as it produces a higher proportion of viable offspring. A large population may carry the recessive deleterious alleles but is unable to give rise to such individuals as those alleles are rarely expressed.