Visible polymorphisms in domesticated animal populations and their role in breed creation: a review

Abstract

Many populations within domesticated animal species have readily observable visible polymorphisms in traits such as color, coat type and horns, among others. these polymorphic populations have commonly been referred to as nonuniform, traditional or primary. the visible polymorphisms are due to several alleles which are segregating at different loci, often in a state of equilibrium as defined by the principles of population genetics. the most usual assumptions surrounding these polymorphic populations are that they belong to an infinite population which is under random mating (panmixia). these two conditions have generally been confirmed by field studies. It is believed that in several animal species these polymorphic populations occurred after domestication and served as a reservoir from which breeders have been developing standardized or fixed breeds (in association with breeding companies or through recognition by official bodies). Polymorphic populations also exist in wild species but they are rare, and obvious polymorphisms are usually only observed in domesticated species which are run in large flocks or herds and in which the control of mating is minimal, namely in some species managed under extensive conditions. the genetic loci which control the visible polymorphisms are mainly coat color loci. Many of these are allelic series which have been well characterized in term of homology, beginning in the early days of Mendelian genetics as a science. these homologies have since been confirmed by molecular genetic investigations.