Equine Coat Color – Red Factor


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The inheritance of basic horse colours black, bay and chestnut is determined by two different gene loci (the Extension and the Agouti locus). The Extension locus (E) corresponds to the melanocortin-1 receptor gene which accounts for the different melanin types. While the dominant allele E is associated with black pigment (eumelanin) the recessive allele e results in reddish coat colour (pheomelanin). Horses showing some black pigment therefore bear at least one copy of allele E at the Extension locus, either in homozygous (EE) or in heterozygous (Ee) state. Horses lacking black pigment (chestnuts) are reddish in colour and homozygous ee. Depending on the genotype of the mating partner, carriers of the red factor (i.e. animals with gene status Ee or ee) can produce reddish coloured offspring. In contrast, animals that are homozygous EE will never produce red offspring, regardless of the colour of the mate.

Red factor testing can be used to differentiate between black horses carrying Ee and those carrying EE. For further information on the inheritance of colours in horses please visit the section about Agouti testing.

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