Why does inbreeding increase the chance that harmful recessive genes will affect offspring?

In the context of animal breeding, inbreeding increases the likelihood that harmful recessive genes will affect offspring because it significantly raises the probability of homozygosity by descent.1

When closely related animals mate, they are more likely to share the same recessive alleles inherited from a common ancestor. In a normal, outbred population, a harmful recessive gene is often masked by a dominant, healthy allele, leaving the individual as a phenotypically unaffected carrier. However, because inbreeding increases the proportion of homozygous offspring, it forces these recessive alleles to pair with identical copies of themselves.2134

When an offspring inherits two copies of a deleterious recessive allele, the harmful trait is no longer masked and becomes expressed in the individual's phenotype. This process often leads to "inbreeding depression," which manifests as reduced fertility, slower growth, increased susceptibility to disease, and higher mortality rates. While breeders sometimes use inbreeding to fix desirable traits or eliminate undesirable ones, the increased expression of these hidden, harmful recessive genes is a major risk that can compromise the health and vigor of the stock.34256