Genetic purging

Genetic purging is the reduction of the frequency of a deleterious allele, caused by an increased efficiency of natural selection prompted by inbreeding.

Purging occurs because many deleterious alleles only express all their harmful effects when homozygous, present in two copies. During inbreeding, as related individuals mate, they produce offspring that are more likely to be homozygous. Deleterious alleles appear more often, making individuals less fit genetically, i.e. they pass fewer copies of their genes to future generations. Put another way, natural selection removes inbred children and their offspring with deleterious alleles from the gene pool, purging the deleterious alleles.

Purging reduces both the overall number of recessive deleterious alleles and the decline of mean fitness caused by inbreeding (the inbreeding depression for fitness).

The term "purge" is sometimes used for selection against deleterious alleles in a general way. It would avoid ambiguity to use "purifying selection" in that general context, and to reserve "purging" to its more strict meaning defined above.