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Home , Black horse, Chestnut (horse color), Equine coat color, Palomino, Roan (color), Roan (horse)

By IRENE STAMATELAKYS

Homozygous 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 has an address—a spe - Agouti , you’re out of luck. pair—or, even better, two—could cific site on a specific . 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 a pair of tobiano or black . the locus name to refer to a gene. registered with the APHA are . 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—, dun, bay 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? 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 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, 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 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 ,” 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 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. 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. 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— or or a C Y S E

red dilute, such as 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 , 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 - [] 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. Thus, blacks tend to have less Brooks. “Mother Nature is good at color, chestnuts more. Modifying Tobiano x 2 proving us wrong as soon as we say genes aside, [minimal tobianos] have The tobiano spotting pattern is a something is ‘always’ a certain way.” the same potential to transmit the TO result of the dominant tobiano allele, It is possible for a horse to have the gene as any other TO horse.” symbolized by TO . As long as the foal tobiano genotype but not the pheno - inherits one TO allele, genetically, he’s type, as we have defined it. In other Bet for value a tobiano. If the parent is heterozy - words, you could get a tobiano that By now, it’s pretty clear why gous for tobiano, there’s a 50 percent doesn’t meet the minimum require - homozygosity has its appeal, at least chance for tobiano offspring. If the parent is homozygous for tobiano, 100 percent of the offspring will be tobiano, virtually eliminating the risk of a solid foal. And that is the allure of Facing the facts homozygous tobianos—a 100 percent Myth: Homozygous tobianos give you a 100 percent color guarantee. color guarantee. Fact: This actually depends on your definition of “color.”The foal will But wait! Before you sign that definitely inherit a tobiano gene, but occasionally it’s expressed only minimally. It’s possible that the foal won’t have enough white hair breeding contract, there are a few over unpigmented skin in a qualifying area to be eligible for APHA’s things you should know. Regular Registry. Although such a foal would be registered as a solid According to researchers, it is impos - Paint-bred, genetic testing could confirm the foal carries the tobiano sible to distinguish visually between allele, which it could pass on to offspring. heterozygous and homozygous tobianos with complete accuracy; the Myth: “Paw prints” are proof a tobiano is homozygous. only way to positively identify a Fact: They are called paw prints, bear paws, ink spots and cat tracks— homozygous tobiano is through testing. small, round, colored spots, often surrounded by a roaned halo, Fortunately, the accuracy of tobiano popping up in the white areas of a tobiano.While they are frequently seen on homozygous tobianos, they do not a guarantee that the testing improved dramatically in recent horse really is. Only genetic testing can accurately confirm a horse years. The old test using blood- is a homozygous tobiano. markers and pedigree analysis was wrong as much as 10 percent of the Myth: A homozygous black horse is always black. time. But in early 2008, researchers Fact: “A horse’s final color results from the interaction of 11 generally from the University of Kentucky’s independent processes,” writes Sponenberg. Because multiple Gluck Equine Research Center pub - alleles affect coat color, a homozygous black may not necessarily be lished the results of a study linking the black in color. Depending on the complete genotype, this horse tobiano gene to a chromosome inver - could actually be bay, grullo, smoky black or even gray, just to give a few examples. sion near the KIT gene. As a result, breeders had a new, more accurate Myth: Breeding to a homozygous black horse guarantees you’ll get a DNA test at their disposal to identify black foal. homozygous horses. Even better, the Fact: The only guarantee when breeding to a homozygous black horse new test was done with hair samples, is you won’t get a “red” foal, meaning sorrel or chestnut. A rather than blood, making it much homozygous black is EE , meaning it has two dominant alleles for more convenient. black-pigmented hair. It will always pass one to its offspring. A sor - Samantha Brooks, Ph.D. (now of rel or chestnut is ee , meaning it has two recessive alleles and does the Department of Animal Science at not have black hair. Depending on the foal’s complete genotype, it could be any of the black-based colors, but never sorrel or chestnut. Cornell University), whose research gave rise to today’s tobiano test, says Myth: Any tobiano or black horse could be homozygous for that gene. that a group in Switzerland published Fact: To get a homozygous tobiano, both parents must be tobiano.The a paper validating the new inversion same rule applies for a homozygous black horse. Both parents must test. There have been zero false posi - be black-based and have an E allele to pass on to the foal.

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Although color should not be the sole basis for a breeding decision, choosing a stallion homozygous for tobiano and/or black—such as Sum - merschicobandito (above) or Strait Texas Kaliman (right)—can help reduce the unpredictability of the resulting foal’s color.

when it comes to the tobiano and black genes. “Tobiano homozygosity helps

keep the color on the babies, and A M U

that’s what people are looking for,” O B Y T

said Miller. “Obviously, homozy - T A P gous tobianos aren’t the only ones D N A L

that produce color, but they are the A E N

only ones that guarantee it.” Y S E T This, she says, is especially impor - R U O tant to owners with solid Paint-bred C and Quarter Horse mares. However, in today’s market, she feels homozy - Ann Stockstill of Winona, Texas, But does homozygosity automati - gosity for tobiano or black is a helpful stands Summerschicobandito and cally generate higher breeding fees? marketing tool, but no longer the Chicos Bandelero, both of whom are Not always, says Stockstill. only criteria mare owners consider. homozygous tobiano, homozygous “I think that a stallion can com - “There are enough stallions out black stallions. She says that homozy - mand a higher breeding fee if he is there that bloodlines and performance gosity is a strong selling point. both homozygous and has an impres - in the show pen are also important. “The basic concern is the guarantee sive performance record,” she said. When you add the homozygosity, it of a colored foal,” Stockstill explained. “Shipping semen to foreign countries helps get your stallion into the mar - “Most of my clients want a black is more in demand if the horse is ket and move up the ladder.” tobiano foal.” homozygous also.”

74 N FEBRUARY 2010 N PAINT HORSE JOURNAL And it helps sell horses in a trou - bled economy, says Stockstill. Test prep “In the market today, my clients in Of the major U.S. laboratories, three offer the Tobiano , Extension other countries are wanting double (also called Red Factor) and Agouti tests using hair samples. homozygous horses with good pedi - grees and prefer that they have sires Laboratory University of University of Animal California, Davis Kentucky Genetics, Inc. and dams with performance records,” she said. Gene vgl.ucdavis.edu ca.uky.edu/gluck/ horsetesting.com AGTRL.asp Neal and Patty Bouma of Choteau, Montana, acquired their first Tobiano $25 $45** $25 homozygous tobiano stallion in 1995. Extension $40* $45** $25*** Today they stand Strait Texas Kali - Agouti $45** $25*** man, who is homozygous for both the *University of California, Davis, only offers the Extension and Agouti black and tobiano genes. tests together for $40. “Black and white still sells better **University of Kentucky offers three tests, ordered at the same time and for more money than any other for one horse, for $35 each. color,” said Neal Bouma. “The pedi - ***Animal Genetics, Inc. offers the and tests Extension Agouti gree is very important too.” together for $40. Guaranteed color and top perfor - mance bloodlines are a combination Tobiano results are reported as: they believe is essential for success. N/N No evidence of altered sequence detected. Horse is not “The demand for double homozy - Tobiano. gous horses is still high and they sell for at least double and even up to four or N/TO One copy of altered sequence. Approximately 50 percent of the offspring will inheritTobiano. five times as much as a colored Paint,” said Bouma. “We are selling more Two copies of altered sequence. Horse is homozygous TO/TO horses for more money than we ever forTobiano. All offspring will inheritTobiano. have. I think it is because of our blood - lines, the way we promote them and Agouti results are reported as: the homozygous factor combined.” A/A or A/a Black pigment distributed in point pattern. The basic color of the horse will be bay or brown in the absence Drawing two pair of other modifying genes. A has no effect on red pig - What are the chances of drawing two ment ( ee ). pair? Well in poker, the probability of being dealt two pair is 4.8 percent. And a Only recessive allele detected. Black pigment distributed uniformly. The basic color of the horse will be black in in Paints? Well, let’s put it this way. If the absence of other modifying genes. you don’t try, you never will. “When you breed to a Quarter Horse, you take away the possibility of Red Factor ( Extension ) results are reported as: having a homozygous tobiano baby,” e Only the red factor detected. The horse can be says Stockstill. If this is to be in the assumed to be homozygous for red ( ee ). The basic cards, then you’ve got to follow the color is sorrel or chestnut, but depending on genes at genetic rules and breed tobiano to other color loci, the horse could be palomino, red dun, tobiano and black-based to black-based. gray, cremello or white. Bouma is hoping for a few double E/e Both black and red factors detected. The horse can be homozygous foals this spring. assumed to be heterozygous for the red factor ( Ee ). It “That would be our ultimate can transmit either E or e to its offspring.The basic color goal,” he said. “The odds are low to of the horse will be black, bay or brown, but depending get it all, but you can’t get it if you on genes at other color loci, the horse may be buckskin, aren’t trying. I have never shot a bull dun, grullo, perlino, gray or white. elk sitting in my living room either. E No red factor detected.The horse can be assumed to be You have to be rolling the dice or homozygous for black pigment ( EE ). It cannot have red playing the lottery to win.” foals, regardless of the color of the mate.The basic color Now that you know the rules, you of the horse will be black, bay or brown, but depending are ready to play. p on genes at other color loci, the horse may be buckskin, zebra dun, grullo, perlino, gray or white. To comment on this article, e-mail Source: UC-Davis Veterinary Genetics Laboratory ( vgl.ucdavis.edu ) [email protected] .

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