H Locus – Harlequin Dog Coat Color
Harlequin dog coat color is a trait among domestic dogs controlled by two loci: the gene responsible for merle coat color (SILV) and a modifier gene of SILV, the harlequin (on H locus). A modifying gene is a gene that influences the expression function of another gene. True harlequin occurs only in Great Danes, but similar coat color pattern can be seen also in Australian Sheepdogs and in Border Collies.
Melanin is a group of natural pigments which are produced in the melanocytes cells. Two basic pigments in the dog’s coat are eumelanin and phaeomelanin. SILV gene and a retrotransposon insertion near its exon boundary are connected to merle coat color. It influences the eumelanin, while phaeomelanin will not alter under SILV expression. Random parts of dog’s coat will appear as diluted (for example grey in black colored dog), while only patches of original color will remain. Truncation of poly(A)-tail will allow normal pigmentation in the dog’s coat color. The harlequin, in this case as a merle modifier, is inherited together with merle and affects the way merle pattern appears. It turns the areas between the dark pigmentated spots on merle into completely white. Sometimes it appears with some grey patches. For example, all grey in blue or black merle dog possesing harlequin gene will turn into white.
Modifier gene H of SILV gene is located at canine chromosome 9. It is a dominant modifier. It will be expressed in the dog’s phenotype only if the dog has merle coat color. Dogs that are non-merle (genotype mm), although they carry H gene modifier, will not have harlequin coat color expressed. Harlequine Great Danes are obligate heterozygotes (genotype Hh). Homozygote genotype HH is lethal and homozygous dogs die embrionically in the womb.
Clark, L. A. (2011.): A missense mutation in the 20S proteasome β2 subunit of Great Danes having harlequin coat patterning. Genomics. 2011 Apr;97(4):244-8. doi: 10.1016/j.ygeno.2011.01.003. Epub 2011 Jan 20.