By IRENE STAMATELAKYS Homozygous tobiano and homozygous black could be winners for your breeding program, if you know how to play your cards. L L I T S K C O T S N N A Y S E T R U O C n poker, a pair is not much to brag gets one of the pair from the sire and the in equine color genetics. If your goal about. Two pairs are just a hair bet - other of the pair from the dam.” is a black foal, and you’ve drawn the ter. But in equine color genetics, a Every gene has an address—a spe - Agouti allele, you’re out of luck. pair—or, even better, two—could cific site on a specific chromosome. be one of the best hands you’ll ever We call this address a locus—plural The Agouti effect hold. We’re talking about a sure bet— being loci. Quite often, geneticists use Approximately 20 percent of horses a pair of tobiano or black genes. the locus name to refer to a gene. registered with the APHA are bay. If Any Paint breeder will tell you that When a gene comes in different you also include the colors derived producing a quality foal that will forms, those variations are called alle - from bay—buckskin, dun, bay roan bring in top dollar is a gamble. In this les. For example, there is a tobiano and perlino—almost one-quarter of business, there are no guarantees. But allele and a non-tobiano allele. Either registered Paints carry and express the what if you could reduce some of the one can occur at the tobiano locus, Agouti allele, symbolized by an upper - risk in your breeding program as well but each chromosome can only carry case A. This dominant allele is respon - as increase the marketability of your one allele at a time. So a horse will sible for restricting black hair to the product, with fewer solid foals and have no more and no less than two points, on horses capable of forming more sought-after colors? alleles for any trait. black pigment. Whether a horse is het - That’s where a horse that is homozy - This leads us to the definition of erozygous ( Aa ) or homozygous ( AA ) gous tobiano and/or homozygous black homozygous, which Sponenberg for Agouti , the result is the same—a could give you a competitive edge, in horse with black points. marketing terms. Add that to an The recessive version of the allele, already attractive package—good symbolized by a lowercase a, is bloodlines and a performance record— For a horse to be black, expressed only in the homozygous and you’ve got a winning hand. state. If a horse is aa , then the black But before you place your bets, his genotype must be hairs—if the horse is capable of form - what exactly does “homozygous” ing black pigment—are spread uni - mean? What should you know before formly over the body. About 16 per - you make any breeding decisions? Eeaa or EEAA ... As long cent of horses registered with the And how important is homozygosity APHA are black, grullo or blue roan. to Paint Horse breeders in today’s as the foal inherits one If you are breeding for a specific market? We’ll answer those questions color, say black, it is important to and more, with a focus on homozy - TO allele, genetically, know the Agouti genotype, or the gous tobianos and blacks, the two you genetic makeup, of your breeding see advertised most frequently. he’s a tobiano. stock. For example, if a homozygous black stallion is AA , then he will Genetics made simple always pass the Agouti allele to his off - Simply put, homozygous is an spring and only throw bay colors. If adjective used in genetics to describe describes as “the condition of having he is Aa , then there is a 50 percent having two identical genes for a par - two identical alleles at a locus (one on chance his offspring will be bay or a ticular trait. If your high school biol - each chromosome).” The opposite derivative of bay. If that stallion is aa , ogy class is a distant memory, let’s would be heterozygous, where the then he will always pass on the reces - quickly review some basic genetics. two alleles are different. sive allele, and his offspring could In his book Equine Color Genetics , As simple as that concept may potentially be a uniformly-distributed author D. Phillip Sponenberg, seem, many people mistakenly use the black-based color, depending on the DVM, Ph.D., explains that genes term, saying “my horse is homozy - dam’s genetic contribution. control all chemical processes occur - gous.” If you know anything about ring in living organisms, including genetics, it should lead you to won - Pure black traits like hair color. der, “homozygous for what?” Perhaps the more correct term is “In horses, as in all mammals, genes That’s because a horse can be “homozygous for black pigment,” but occur on chromosomes,” writes Spo - homozygous for any number of alle - that would make for some very wordy nenberg. “Chromosomes can be les: Agouti, black, champagne, cream, ad copy. A homozygous black horse is thought of as strings of genes. Chromo - dun, gray, pearl, red, silver, Sabino 1 not necessarily black, nor does he somes occur in pairs, and an individual and tobiano, just to name a few throw 100 percent black offspring. equine coat color characteristics. The only guarantee here is this horse Homozygosity for black and/or tobiano But first, let’s take a look at the is not red and will not produce red can give your horses a marketing edge. Agouti allele, which plays a critical role offspring. Why is that? PAINT HORSE JOURNAL N FEBRUARY 2010 N 71 L L I T S K C O T S N N A Y S E T R U O C Above: “Ink spots,” like those on Chicos Bandelero’s coat, aren’t proof of homozygosity. Genetic testing is the only way to know—this stallion is homozygous for black and tobiano. Right: The bay stallion Sacred Assets is homozygous for black and tobiano, so he’ll never throw a red-based foal. To understand this complex con - cept, you have to understand the workings of the Extension locus, and its interaction with the Agouti locus. The recessive version of the Exten - sion allele, symbolized by a lowercase e, is expressed only in the homozy - R E L gous state. This is commonly called L I M L the “red factor.” If a horse is ee , then it Y R E H will be red—sorrel or chestnut or a C Y S E red dilute, such as palomino or red T R U dun. And if the horse is homozygous O C , E D ee , this will completely mask the I R B K R Agouti genotype. You cannot tell, just I K F F by looking, if a homozygous ee horse E carries one or more Agouti alleles, and J therefore could produce a bay foal which would make him bay. Or, he black-based horse, for example a bay, to when bred to a black-based horse. could have one or more dilution genes, be homozygous black. Simply, the The opposite of red-based, in equine which would modify the color even expression of that black Extension allele coat colors, is black-based, symbolized further, making him buckskin, grullo, depends entirely on the Agouti allele. by an uppercase E. This is also called smoky black, or dun—just to list a few Sacred Assets is one such example. the “black factor.” Whether the horse is possibilities. The 2005 bay stallion is homozygous heterozygous Ee , or homozygous EE , For a horse to be black, his genotype for the black gene, as well as the the base color of the coat is black. must be Eeaa or EEaa . The only differ - tobiano gene. Owner Cheryl Miller of However, several variables can modify ence is that a homozygous black horse Kaleva, Michigan, says she does get that black base color. For example, if (EE ) will always pass that black factor to questions about this. the horse is an Agouti carrier, the black his offspring, guaranteeing that they will “I’ve been asked, ‘If he’s bay, how hair will be restricted to the points, be black-based. But it is possible for any can he be black?’” said Miller. “But 72 N FEBRUARY 2010 N PAINT HORSE JOURNAL bay does come from the black gene.” tives and negatives to date with the ments to make it eligible for the She explains to them that it simply new test, says Brooks. APHA’s Regular Registry. means he’ll never throw a red foal. Although the new test is highly “I have seen some of these minimal “There’s nothing wrong with sorrel, accurate, there’s still the possibility a tobianos,” Brooks said. “There may but there are a lot of sorrels out there,” homozygous tobiano could throw a be other genes modifying the extent Miller said. “People are interested in solid foal. How so? of white. For example, the MC1R getting something different. They “Expression of the tobiano pheno - [melanocortin 1 receptor] and Agouti don’t know there is a homozygous type [external appearance] is variable genes both have effects on the extent black, that he’ll only throw those and sometimes does not conform to of white in patterns, including black-based colors.” the definitions we like to give it,” said tobiano.