The S-Series Locus – White Spotting Pattern This is a very interesting locus affecting the distribution pattern of coloured or of white areas on the body surface. There are 4 established alleles – • S for solid colour – refers to self-coloured – or solid colour with little or no white • si for irish spotting – variable white on the face, chest, underbody, lower legs and feet and tail tip • sp for piebald – more than half white, it appears to have large coloured spots on a white background. • sw for extreme white – completely white, it may have some colour markings but these are confined to the head Each of these alleles causes a certain amount of coloured and white areas and these fluctuate around an average colour distribution for each allele which the main gene tries to produce. Unfortunately this is a particularly complicated locus due to the effect of genetic modifiers. Modifiers are genetic factors independent of the main gene that act on the expression of the main gene. In this particular case, there are modifiers called ‘plus’ modifiers which cause more pigment in the phenotype than is expected for the genotype and there are modifiers called ‘minus’ modifiers which cause less pigment in the phenotype than expected for the genotype. This results in it being almost impossible to discern genotype from phenotype except in the case of the extreme white dog which would be swsw. Even in this case, minus modifiers cause the dog to be completely white whereas plus modifiers cause coloured patches on the head. It is believed that the sp piebald allele is not found in Bull Terriers any more. The following diagram is an adaptation of a similar diagram for colouration in the Basenji in Clarence Little’s 1957 book “The Inheritance of Coat Color in Dogs” showing how individuals with the same genotype – in this case sisi – can have such variable phenotypes due to the actions of modifiers. PLUS MODIFIERS MINUS This means that there is huge overlap in phenotype between different genotypes. Although the above dogs all have the genotype sisi, those towards the left would look the same as dogs with the genotypes SS, Ssi and those dogs towards the right would look the same as dogs having the genotypes Ssw and sisw. Thus the genotypes SS, Ssi, sisi, Ssw and sisw are indistinguishable from each other due to the action of the plus and minus modifiers. When looking at Bull Terriers, there are clearly three colour patterns as far as white markings are concerned: all white (with patches of colour on the head in some cases), strong white markings on the nose, chest, collar, neck, underbelly, lower legs and tail tip and minimal white markings (most likely on the toes, chest or nose). To simplify any genetic analysis we can assign the following genotypes to these phenotypes: Genotype Phenotype Comment si si Little or no white confined to the blaze, chest and toes This is the case in the so-called Solid Bull Terrier si sw White blaze, chest, collar, underbelly, lower legs and feet and tail tip, white may only cross the back of the body at the neck This is the case in the Coloured & White Bull Terrier sw sw All white – may have some colour markings confined to the head This is the case in the so-called White Bull Terrier We may be excluding the S allele and the SS, Ssi and Ssw genotypes but as they are indistinguishable from sisi and sisw genotypes when expressed, this has little effect on our genetic analyses. Rather it is the significant influences of the plus and minus modifiers on all of the alleles of this S-series gene and their possible genotypes that cause confusion between Solid-Coloured and Coloured & White Bull Terriers. Sometimes a genetically Solid-Coloured dog can have quite significant white on the chest – it proves it is genetically Solid-Coloured by an inability to produce White puppies. Alternatively, a dog with hardly any White marking can unexpectedly produce a White puppy thereby demonstrating that genetically it is actually Coloured & White. The sw sw genotype is the one that James Hinks was originally breeding for in the 1800s. The effect of this genotype is a white mask over a Coloured Bull Terrier. All White Bull Terriers still have colour genetically and can pass these colours on to their offspring. As all White Bull Terriers have this double recessive, when bred with each other they will always only produce White puppies. It is interesting that when this white marking is on the muzzle for example in the case of a Coloured & White Bull Terrier, it can hide the smut markings caused by the E-Series Locus. Sometimes just a portion of the smut mask is visible forming a black lip marking or tear marks on the sides of a white blaze.s
Posted on: Fri, 04 Oct 2013 19:03:10 +0000
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