21/08/2017
Origins and inheritance of the Pygmy Goat
Dr Natalie Warnick (BVSC Hons)
It is believed that the pygmy goat in the USA (from which the original Australian genetics were sourced) is descended from goats imported from several West African countries such as Cameroon, Nigeria, Ghana and Guinea - via European zoos and directly - referred to by many names including the West African Pygmy and Cameroon Dwarf Goat. Unfortunately, it appears impossible to provide more definitive information on the exact origins of the breed in the USA from the available scientific literature.
Dwarfism in animals is often genetically controlled via simple inheritance with a single pair of alleles, however this does not appear to be the case in Pygmy goats. Although dwarf traits in goats are inherited, the genes responsible for these traits in different populations may not be the same, leading to several different theories over time. It has been postulated that the diminutive size in certain populations of West African Pygmy goats could be the result of inheriting a form of chondrodysplasia such as achondroplasia or possibly pituitary dysfunction.
In his review of congenital abnormalities in goats in 1993, Basrur describes the pygmy goat as an achondroplastic dwarf characterized by short legs, cranial base, and vertebral column due to limited interstitial growth of epiphyseal, articular, and basocranial cartilage. Basrur suggests these traits are expressed due to repeated gene mutations at particular loci or mimic genes. Maurice Shelton, in work on goat reproduction and breeding in 1973, described the appearance of many pygmy goats in the USA as relatively normal in proportion, apart from the small size, so less likely to be simple achondroplastic dwarfs.
In a study of the morphological, physiological and genetics of pygmy goats in the USA in 1971, Blanks postulated that the pygmy is an achondroplastic dwarf with traits controlled by at least 3 genes plus other modifier genes. They were described as homozygous (recessive) for small stature and meatiness (dominant) with a third gene lacking dominance. In this US study, pygmies were crossed with swiss types (who have genes dominant for increased height and recessive for meatiness) to produce an intermediate height 50% buck or doe. The theory was probably that these offspring could then be crossed with others of similar genotype to enable easier selection of those goats with the homozygous pygmy height genes estimated to occur at a ratio of 9:3:3:1 .
This hypothesis was further examined by Simms study of genetic and physiological relationships of growth hormone and growth in goats in 1974 using pygmy goats sourced from the Oregon State University, F1 and F2 crosses. Simms study concurred with Blanks conclusion that more than one gene was involved but ruled out a simple mode of inheritance in the study group stating that polygenic inheritance was more likely - occurring over time due to natural and (more recently) artificial selection affecting several pairs of genes rather than a single mutation at one or two loci. Simms also found that the expression of reduced size in the pygmy goats in his study was not due to a deficiency in the pituitary growth homone (GH) - effectively ruling out pituitary hypoplasia - however the results did suggest a defect in GH utilisation at the cellular level in the pygmy goats. Unfortunately he too failed to find definitive information on the origin of the USA version of the African pygmy goat in the literature.
The current entry in the OMIA (Online Mendelian Inheritance of Animals) DB lists the mode of inheritance of caprine dwarfism as unknown and the genes and mechanisms that have a role in the inheritance of these traits in goats are still under debate
