Brooks Equine Genetics Lab

Coat Color Genetics

Project Description

Coat color has been a subject of interest among horse owners for many years, likely dating back to the start of domestication. While much interest in coat color is due to aesthetics, color genes can also affect a horse’s health. Two examples of diseases that are associated with coat color are Multiple Congenital Ocular Abnormalities (MCOA) with the Silver coat color, and Lethal White Overo foal syndrome with the Frame Overo pattern. Research in other species has examined physiologic traits that vary with coat color, like response to pain and levels of stress hormone in the body.

Much of the color research in our lab focuses on white patterns, especially “new” patterns. When two horses with minimal white markings produce a foal with over half of its body covered in white markings, a new cause of dominant white patterning is often suspected. In other cases, it may be that both parents had a lowly expressed pattern and passed it on, allowing the pattern to “light up” the offspring.

If you have a horse with an interesting coat color or pattern that you would like to include in our DNA bank for possible use in future studies, please contact us.

Coat Color Descriptons

• Base Colors

  Extension (Black versus Chestnut)

  Extension determines whether a horse will be black-based or chestnut-based. Black horses are uniformly dark all over their bodies, with the shade ranging from a sun-faded brown to jet black. Chestnut horses are uniformly red, with the shade ranging from a light orange to even a dark near-black. Chestnut horses may have a much lighter mane and tail (flaxen chestnut).

  Black (E) is inherited dominantly to chestnut (e). There are two currently known chestnut alleles. The first, e, is the result of a C to T missense mutation at codon 83 in the MC1R gene, resulting in a serine being replaced with a phenylalanine (Marklund et al. 1996). The second, ea, is the result of a G to A missense mutation at codon 84 in the MC1R gene, resulting in an aspartic acid being replaced with an asparagine (Wagner and Reissmann 2000).

• Modifiers

  Agouti (Bay versus Black)

  Agouti determines whether a black horse will be bay or black. There is no known affect of agouti on chestnut-based horses. Bay horses are a reddish-brown on most of their body with black legs, ear tips, mane, and tail (points). The non-black area can range from a light brown to near-black. Black horses are uniformly dark all over their bodies, with the shade ranging from a sun-faded brown to jet black.

  Bay (A) is inherited dominantly to black (a). The black allele is an 11 base pair deletion in the second exon of the ASIP gene, believing to extend the transcribed region by 402 base pairs (Rieder et al. 2001).

  Grey (Pregressive greying)

  Grey determines whether a horse will gradually lose color in their coat as they age. Non-white areas in the coat will become progessively lighter, sometimes to the point of appearing completely white. The rate of lightening and the areas affected is variable – some horses retain darker points or have patches of hair that never grey.

  Grey (G) is inherited dominantly to non-grey (g). The grey allele is a 4.6 kbp duplication in intron 6 of the STX17 gene. Although the mechanism is not fully understood, the mutation results in an upregulation of both STX17 and NR4A3 that is believed to cause hyperproliferation of melanocytes leading to premature depletion of melanocyte stem cells (and thus inability to produce pigment) (Rosengren Pielberg et al. 2008).

• Dilutions

  Cream (Bay: Buckskin or Perlino, Black: Smoky Black or Smoky Cream, Chestnut: Palomino or Cremello)

  Cream dilutes the color of red pigment when one copy is inherited and all pigment when present in two copies. When one copy is present, the body color on a bay or chestnut becomes a light tan or gold. The ear tips, legs, mane, and tail on a buckskin remain black whereas the mane and tail of a palomino become white. Although eyes may also exhibit some lightening, the skin on these horses remains black. One copy is usually not apparent on a black horse due to the minimal number of red hairs in the coat. When a horse of any base color inherits two copies, the entire body including mane and tail becomes a light cream color. Eye and skin color are both affected, with eyes becoming a light blue and skin appearing pink.

  Cream (CCR) is inherited in an incompletely dominant fashion to non-cream (C). The cream allele is a G to A missense mutation in codon 153 of the MATP/SLC45A2 gene, resulting in an aspartic acid residue being substituted for an asparagine (Mariat et al. 2003)

• Patterns

  Dominant White (Dominant White versus Solid)

  Dominant white is a collection of white spotting patterns characterized by up to 100% white skin and hair coat across the horse from birth. In some individuals, it can have a similar appearance to sabino spotting (see below). Unlike most of the other colors described, dominant white has many different underlying genetic causes present in different family lines. Dominant white is frequently suspected when a predominantly white foal is born to two solid parents, especially when the horses have tested negative for known white spotting patterns.

  Dominant white (W) is inherited dominantly to non-white (+). Studies in other species suggest that W alleles may be embryonic lethal and for some alleles in the horse this is likely also the case.  Currently, there are over 30 unique dominant white alleles reported in the literature, all disrupting function of the KIT gene.

  Frame Overo (Frame Overo versus Solid)

  The frame overo spotting pattern is characterized by white spotting that is “framed” with color, usually arranged horizontally. The white areas in a horse with only frame patterning rarely crosses the topline. Frame overos may have one or two blue eyes. Expression of the pattern can range from minimal body white, sometimes with blue eyes, to white spotting on more than half of the body. Horses with two copies of the frame overo mutation have a condition known as lethal white foal syndrome, characterized by almost no pigment in the coat and an inability to pass feces. These foals are unable to survive and should be humanely euthanized. Genetic testing for the frame overo gene is important to avoid producing lethal foals by never crossing two frame overo carriers.

  Frame overo (O) is inherited in an incompletely dominant fashion to non-white. The frame overo allele is a TC to AG substitution in codon 118 of exon 1 of the EDNRB gene, resulting in an isoleucine being replaced with a lysine (Metallinos et al. 1998, Santschi et al. 1998, Yang et al. 1998).

  Please note that all homozygous frame overo (O/O) offspring will not survive.

  Sabino-1 (Sabino Overo versus Solid)

  Sabino is a white spotting pattern in horses usually characterized by white patches on the face, lower legs, or belly and interspersed white hairs on the midsection. Outward differences in appearance between horses with two copies of the gene (homozygotes) and those with only one (heterozygotes) can be quite striking with some homozygous individuals appearing completely white.

  Sabino-1 (SB1) is inherited in an incompletely dominant to non-sabino (N). The sabino-1 allele is a T to A substitution located 13 base pairs upstream from KIT exon 17, resulting in mRNA transcripts lacking exon 17. Homozygous (SB1/SB1) horses retain some transcripts with exon 17, though still significantly less than heterozygotes (Brooks et al. 2005).

Additional Information

• Published Articles
  • Articles (.pdf 245KB)
• Punnet Squares

  The following tables represent the predicted offspring of various crosses of parents. Although crosses are only shown for one of the possible traits, the same information applies for each trait given by “Example traits.” The headings of each table give the outward appearance of the parents whereas the the labels within the tables identify the genotype. Below the table are the percentages of each type of offspring you would expect to see if you crossed the same parents many times. Due to the number of offspring horses produce, you will likely not see these exact ratios. Essentially, you are flipping a coin with each mating – if you only flip a coin 10 times, it is likely you will not see 5 heads and 5 tails, though you would see nearly half and half if you flipped the same coin a thousand times.

  • Punnet Squares