Locus I

The locus I affects the expression of the pheomelanin pigment. Beside the eumelanin, the pheomelanin is one of the main pigments. While the eumelanin is a dark pigment causing black coat colour and its dilution results in brown or blue colour, the pheomelanin is a red pigment. The extreme dilution of the pheomelanin is caused by a mutation in I-locus (c.151C>T in MFSD12-gene). The mutation of MFSD12 gene does not affect the eumelanin so that the black coat and the black hair ends remain black.

The mutation affects the pheomelanin in the entire coat and its total absence or the presence of only a minor amount of this pigment causes a pure white or cream colour of dog´s coat. However, the pigmentation of the nose, lips, eyes and skin remains unchanged, so we cannot say that these dogs are albinos.  As it is not a monogenic trait the intensity of the dilution may differ in various dog breeds.

In brindled dogs, the homozygous mutation in the locus I is expressed by lighter red, however the black colour remains unchanged. It is the same with black and tan dogs, where the mutation expresses itself in lighter tanning only. The while coat colour (e.g. in Samoyeds, White Swiss Shepherds, America Eskimo Dogs, Poodles, Bichons, Maltese dogs and many other dog breeds) occurs when there is a combination of extreme dilution of the pheomelanin and the absence of eumelanin, which is  however caused by other mutation (genotype e/e in MC1R gene, i.e. locus E).

The colour dilution caused by locus I is inherited autosomal-recessively. This means that the phenotype of diluted colour is expressed only in homozygous individuals with genotype i/i. It is necessary that the dog inherits this mutation from each of dog´s parents.

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Reference:

Hédan et al., Hédan, B., Cadieu, E., Botherel, N., Dufaure de Citres, C., Letko, A., Rimbault, M., Drögemüller, C., Jagannathan, V., Derrien, T., Schmutz, S., Leeb, T., André, C.: Identification of a Missense Variant in MFSD12 Involved in Dilution of Phaeomelanin Leading to White or Cream Coat Color in Dogs. Genes (Basel) 10:386, 2019.