Genetic test that analyzes the main coat color dilution variants in the horse. Dilution lightens the pigment (eumelanin and/or pheomelanin) without producing pure white and explains phenotypes such as cream, pearl, champagne, silver dapple, mushroom and dun. It is an orientative test of the phenotype; complementary to, not a substitute for, phenotypic assessment and clinical examination.

Applicable breed: multiple breeds and ponies. Population frequencies: limited data; the distribution varies greatly between breeds and there are no validated frequencies for all variants.

• Cream (C^Cr): lightens pheomelanin in heterozygosity (palomino/buckskin) and both pigments in homozygosity (cremello/perlino).\n• Pearl (C^prl): recessive dilution; with one cream allele it produces a double-dilute-like phenotype.\n• Sunshine and snowdrop: recessive SLC45A2 dilutions in horses that appeared double-diluted.\n• Champagne (SLC36A1): lightens hair and skin, with light eyes and light hooves.\n• Silver dapple (PMEL): lightens the mane and tail on black coats.\n• Mushroom (MFSD12): dilutes the chestnut coat toward a sepia tone.\n• Dun (TBX3): dilution with primitive markings (dorsal stripe and leg bars).

Panel of dilutions described at different times: cream (SLC45A2, 2003) and later pearl, sunshine and snowdrop in the same gene (2019-2020); champagne (SLC36A1, 2008); silver dapple (PMEL, 2006); mushroom pheomelanin dilution (MFSD12, 2019); and dun with primitive markings (TBX3, 2016).

• Do not pair two animals carrying the same recessive dilution (e.g. pearl, mushroom) unless the homozygous phenotype is sought.\n• For cream, avoid cream × cream crosses unless a double dilution is intended; the double dilute may be associated with auditory or visual problems in some lines.\n• For silver, consider the pleiotropic effect on vision (MCOA) in homozygotes and do not select on color alone.\n• A negative result for one variant does not rule out other dilutions not included in the panel.

Interpretation must be made on the complete phenotype, since the dilution genes interact with each other and with the extension and agouti loci. For silver, assess ophthalmological examination in homozygotes (MCOA). For cream, assess audiometry if there is suspicion of hearing loss in double dilutes.

1. Mariat 2003, A mutation in the MATP gene causes the cream coat colour in the horse (PMID 12605854); OMIA:001344-9796.
2. Sevane 2019, Missense mutation in SLC45A2 causing pearl coat dilution (PMID 30968968); OMIA:001344-9796.
3. Holl 2019, SLC45A2 variants explain pearl and sunshine (PMID 31006892); OMIA:001344-9796.
4. Bisbee 2020, SLC45A2 variant associated with snowdrop (PMID 31961951); OMIA:001344-9796.
5. Cook 2008, Missense mutation in SLC36A1 responsible for champagne (PMID 18802473); OMIA:001263-9796.
6. Brunberg 2006, Missense mutation in PMEL17 associated with Silver (PMID 17029645); OMIA:001438-9796.
7. Tanaka 2019, Frameshift variant in MFSD12 explains mushroom (PMID 31635058); OMIA:002197-9796.
8. Imsland 2016, Regulatory mutations in TBX3 underlies Dun (PMID 26691985); OMIA:001972-9796.