ABSTRACT
The genetic consequences of intense inbreeding without selection have long been known to be disadvantageous. Inbreeding leads to a depression in performance of most traits; causes deterioration in constitutional vigor and health; decreases fertility; and increases lethals and physical abnormalities. In practice, horse breeders have noticed that some families of horses can apparently withstand continued close breeding—mild inbreeding or linebreeding—with no noticeable detrimental effects, while other families cannot exist under a similar breeding program. The mating of full sibs represents a much more intense system of inbreeding than does the mating of half-sibs. Parent-offspring matings are equal to full-sib matings in intensity of inbreeding. Homozygosity, or the fixation of genes in double doses, is the effect of inbreeding, and heterozygosity is the distinguishing feature of hybrid vigor. By changing heterozygotes to homozygotes, inbreeding brings to light many of the recessive genes that would otherwise remain hidden.
