Replacing a detrimental allele. (A) A purebred dog from a breed carrying the dominant functional allele is crossed to a purebred dog from a breed homozygous for an inactivating mutation (P1 cross). Progeny of this cross carrying the normal allele are selected and crossed to purebred animals from the breed of interest. In each generation, those carrying the normal allele are chosen for breeding to purebred animals. Eventually it would be possible to mate two animals that are both heterozygous for the desired allele and produce homozygous progeny, with very little change in genetic composition (B) After five generations of backcrossing to the desired breed, the proportion of the genotype contributed by this breed is 96.9%, while by ten generations it is 99.9%.