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Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Bay is the most common color of horse, followed by black and chestnut. A change at the agouti locus is capable of turning bay to black, while a mutation at the extension locus can turn bay or black to chestnut. These three "base" colors can be affected by any number of dilution genes and patterning genes. The dilution genes include the wildtype dun gene, believed to be one of the oldest colors extant in horses and donkeys. The dun gene lightens some areas of the horse's coat, while leaving a darker dorsal stripe, mane, tail, face, and legs. Depending on whether it acts on a bay, black, or chestnut base coat, the dun gene produces the colors known as bay dun, grullo, and red dun. Another common dilution gene is the cream gene, responsible for palomino, buckskin, and cremello horses. Less common dilutions include pearl, champagne, and silver dapple. Some of these genes also lighten eye color. Genes that affect the distribution of melanocytes create patterns of white spotting or speckling, such as in roan, pinto, leopard, white or white spotting, and even some white markings. Finally, the gray gene causes depigmentation of the hair shaft, slowly adding white hairs over the course of several years until the horse's body hair is near or completely white. Some of these patterns have complex interactions. For example, a single horse may carry both dilution and white patterning genes, or carry genes for more than one spotting pattern. Horses with a gray gene can be born any color and their hair coat will lighten and change with age. Most wild equids are dun, as were many horses and asses before domestication of the horse. Some were non-dun with primitive markings, and non-dun 1 is one of the oldest coat color mutations, and has been found in remains from 42,700 years ago, along with dun. Non-dun 2, the version of the dun gene that most domestic horses have, is thought to be much more recent, possibly from after domestication. Leopard complex patterns also predate domestication, having been found in horse remains from 20,000 years ago. The mutation responsible for black and grullo also predates domestication. The mutations causing chestnut, sabino 1, and tobiano appeared shortly after horse domestication, roughly 5000 years ago. Silver and cream dilutions appeared at least 2,600 years ago, and pearl appeared at least 1400 years ago. The gray mutation is also post-domestication but thought to be thousands of years old as well. Fundamental concepts Terminology Heritable characteristics are transmitted, encoded, and used through a substance called DNA, which is stored in almost every cell in an organism. Proteins are molecules that do a variety of different things in organisms. The DNA instructions for how to make a protein are called a gene. A change to the sequence of DNA is called a mutation. Mutations are not inherently bad; genetic diversity itself ultimately comes from mutations. Mutations that happen within a gene create alternate forms of that gene, which are called alleles. Alleles of a gene are simply slightly different versions of the instructions on how to make that gene's protein. The term "allele" is sometimes replaced with the word "modifier", because different alleles tend to modify the horse's appearance in some way. DNA is organized into storage structures called chromosomes. A chromosome is simply a very long piece of DNA, and a gene is a much shorter piece of it. With some rare exceptions, a gene is always found at the same place within a chromosome, which is called its locus. For the most part, chromosomes come in pairs, one chromosome from each parent. When both chromosomes have the same allele for a certain gene, that individual is said to be homozygous for that gene. When the two alleles are different, it is heterozygous. A horse homozygous for a certain allele will always pass it on to its offspring, while a horse that is heterozygous carries two different alleles and can pass on either one. A trait that is only expressed when the gene is homozygous for its allele is called recessive, and a trait that has the same effect no matter whether there is one copy or two is called dominant. Notation Often, the dominant allele is represented by an uppercase letter and the recessive allele by a lowercase letter. For instance, in silver dapple, this is Z for the dominant silver trait and z for the recessive non-silver trait. However, sometimes the alleles are distinguished by which is the "normal" or wild type allele and which is a more recent mutation. In our example z (non-silver) would be wild type and Z would be a mutation. Wild type alleles can be represented as + or n, so Zz, Zz+, Z/+, and Z/n are all valid ways to describe a horse heterozygous for silver. Wild type notation is mainly useful when there is no clear dominant/recessive relationship, such as with cream and frame overo, or when there are many alleles on the same gene, such as with MITF, which has four known alleles. Using n is also common in the results of genetic tests, where a negative result usually means none of the known mutations were found, but does not rule out undiscovered mutations. Melanin Genes affecting coat color generally do so by changing the process of producing melanin. Melanin is the pigment that colors the hairs and skin of mammals. There are two chemically distinct types of melanin: pheomelanin, which is a red to yellow color, and eumelanin, which is brown to black. Melanin is not a protein and therefore there is no gene that changes its structure directly, but there are many proteins involved in the production of melanin or the formation of melanocytes during embryonic development. Mutations that change the structure of proteins with a role in melanin production can result in slightly different variations of melanin. Some genes do not alter the structure of melanin but instead affect where and whether it is produced. Extension and agouti The genes extension and agouti together affect the placement of the two types of pigment, black eumelanin and "red" (coppery brown) pheomelanin. The extension gene codes for a molecule called the Melanocortin 1 receptor, or MC1R. This receptor straddles the membrane of pigment cells, and when activated it signals the cell to produce black pigment instead of red. A recessive mutation to extension removes this functionality, causing the solid red color of chestnut horses. Extension does not affect skin color. The dominant, wildtype, allele of extension is called E, and the non-extension mutation is called e. Extension is epistatic to agouti, meaning that if a horse has two e alleles, it will be chestnut no matter what genotype it has at agouti. The agouti gene codes for a molecule called the agouti-signaling protein, or ASIP. This molecule interacts with MC1R, the receptor coded by ex.... Discover the Thomas Reissmann popular books. Find the top 100 most popular Thomas Reissmann books.