The appaloosa pattern (the spotted, blanket, and roaning phenotypes collectively associated with the Appaloosa breed and the LP locus) has a confirmed molecular basis in the TRPM1 gene. It is the only major horse coat pattern where the causal gene also produces a sensory deficit: horses homozygous for the leopard complex allele (LP/LP) are night-blind. The same gene, in the same tissues, produces the pattern and the visual impairment. This makes the LP complex an unusually instructive case for understanding how coat patterning genes can have off-target effects, a phenomenon also seen in the IKBKG mutation underlying incontinentia pigmenti and in the EDNRB frame overo lethal white syndrome.
The LP allele and TRPM1
The leopard complex (LP) locus was mapped to equine chromosome 1 (ECA1) by Sponenberg in 1982 and later refined by linkage studies. The causal gene was identified as TRPM1 (transient receptor potential cation channel, subfamily M, member 1) by two independent groups in 2013. Bellone RR, et al., PLoS One, 2013;8(7):e68195. doi:10.1371/journal.pone.0068195 demonstrated that an insertion in the promoter region of TRPM1 disrupts normal gene expression in melanocytes and retinal cells in horses carrying the LP allele. The insertion is a retrotransposon-derived element that alters transcription factor binding, reducing TRPM1 expression in the specific cell types where it is normally active.
TRPM1 encodes a calcium-permeable ion channel expressed in melanocytes (where it affects pigmentation) and in the retinal ON-bipolar cells of the eye (where it is essential for the transmission of visual signals from rod photoreceptors in low-light conditions). Reduced TRPM1 expression in melanocytes produces the characteristic coat dilution and spotting; reduced expression in the retina impairs night vision. OMIA:001890-9796 (Congenital stationary night blindness, Equus caballus) records the causal variant as confirmed for the visual phenotype; OMIA:000301-9796 records the appaloosa coat pattern association.
Genotype and phenotype: LP and PATN1
The LP complex requires at least two loci to explain the full range of appaloosa phenotypes. LP (the TRPM1 allele) is necessary but not sufficient for the leopard (spotted) phenotype; a second, unlinked locus called PATN1 modifies the extent of patterning. Horses with one copy of LP and no PATN1 modifier show a “varnish roan” or “blanket” phenotype: a speckled or blanket effect concentrated over the croup, without discrete spots. Horses with LP plus one or two copies of PATN1 show more extensive spotting (leopard or near-leopard patterns). The interaction between LP and PATN1 is the primary determinant of how spotted an appaloosa-pattern horse appears. Druml T, Seltenhammer MH, Curik I, et al., Mamm Genome, 2013 documented the PATN1 effects in Noriker horses.
Homozygosity for LP (LP/LP) produces both more extensive white patterning and complete congenital stationary night blindness (CSNB). The horse is not otherwise impaired but cannot see in low-light conditions. Because LP/LP horses are produced predictably from LP/+ x LP/+ crosses, the night-blindness risk is relevant to breeding decisions in appaloosa-heavy programs. A DNA test identifying LP zygosity is commercially available and allows breeders to predict both coat pattern extent and CSNB risk in foals.
Physical features of the LP complex phenotype
The appaloosa coat pattern includes several visually consistent features that persist regardless of base coat color:
- Mottled skin: irregular pink-and-dark patches on unpigmented skin around the muzzle, eyes, and genitalia. Visible in foals and stable throughout life.
- Striped hooves: vertical dark-and-light striping on the hoof wall. Not diagnostic on its own (other patterns can produce it) but consistent in LP-carrying horses.
- White sclera: the area surrounding the iris is white, giving the eye a more human-like appearance. Present in most LP carriers.
- Sparse mane and tail: thinner, less dense mane and tail hair than non-LP horses, a feature especially visible in the Knabstrupper and other appaloosa-type breeds.
These secondary characteristics, particularly mottled skin and white sclera, appear even in minimally expressed LP horses and are used by experienced appraisers as a supplementary check when coat pattern alone is ambiguous. The Appaloosa Horse Club requires documentation of at least one of these features (plus coat pattern) for registration as an appaloosa.
Why appaloosa spots are not brindle stripes
Appaloosa spots and brindle stripes are occasionally conflated in informal horse description, particularly in breeds that carry both coat color variation and complex patterning. The distinction is mechanistic and visual. Brindle produces vertical stripes that follow Blaschko’s lines (the developmental migration paths of skin cells) and is either non-heritable (chimeric brindle, somatic mosaicism brindle) or heritable through the BR1 (MBTPS2) X-linked variant. Appaloosa spots are patches of depigmented skin produced by TRPM1 downregulation during embryonic development, with a genetic basis entirely different from any of the brindle mechanisms. A spotted appaloosa horse can be confirmed by LP genotyping; a brindle horse does not carry LP alleles and does not show mottled skin or white sclera.
Sources
- Bellone RR, Brooks SA, Sandmeyer L, Murphy BA, Forsyth G, Archer S, Bailey E, Grahn B. Differential gene expression of TRPM1, the potential cause of congenital stationary night blindness and coat spotting patterns (LP and PATN1) in the horse (Equus caballus). Genetics. 2008;179(4):1861-70. PubMed.
- Bellone RR, Holl H, Setaluri V, Devi S, Maddodi N, Archer S, et al. Evidence for a retroviral insertion in TRPM1 as the cause of congenital stationary night blindness and leopard complex spotting in the horse. PLoS One. 2013;8(7):e68195. PMC3714269.
- OMIA:001890-9796: Congenital stationary night blindness, Equus caballus. Accessed 2026-06-04.
- OMIA:000301-9796: Coat colour, leopard complex, Equus caballus. Accessed 2026-06-04.
- Sponenberg DP, Bellone R. Equine Color Genetics. 4th ed. Wiley Blackwell; 2017. pp. 161–200 (appaloosa and LP complex).