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|The Genetics of Coloration in Texas Longhorns|
© David M. Hillis, Double Helix Ranch
Professor, University of Texas at Austin
This article discusses the genetics
of Texas Longhorn coloration, as it is currently understood. We are continuing
to study this subject and find new genes that are involved in coloration,
and new information is always welcome. If you have comments or questions
about this page, please e-mail
me. For more detail, please see the extended five-part series on the
Genetics of Coloration in Texas Longhorns,
starting in the June 2004 issue of Texas Longhorn Trails.
[Author's note: This article is intended for a general audience of Texas Longhorn breeders, rather than a technical audience. However, some scientific jargon is unavoidable, so if any of the terms are unfamiliar, please see the Glossary. If you have suggestions for other terms that should be in the glossary, please e-mail me. If the article is not clear to you, then I want to know where and why, so that I can fix it.—DMH]
The diversity of Texas Longhorn coloration is celebrated by many modern longhorn breeders. Texas Longhorns also represent one of the best breeds in which to study the genetics of coat coloration, since much of the color variation known to exist in cattle exists in Texas Longhorns. However, this great diversity of color leaves many people confused. I have heard some people say that they selected a bull because it exhibited a particularly desirable color pattern, but then found that none of its offspring seemed to exhibit the same coloration. On the other hand, one Texas Longhorn breeder is well known for advertising a bull that he guarantees to produce calves with black coloration. This page briefly reviews what we do and don’t know about the genetics of color inheritance in cattle, and in Texas Longhorns in particular.
Given the diversity of colors seen in Texas Longhorns, many people are surprised that there are just two different pigments that produce all the hair colors in cattle (and for that matter, all mammals). These two pigments are eumelanin (black) and phaeomelanin (red). Eumelanin is a black pigment, but also looks brown in lower concentrations. Phaeomelanin is a red pigment, but can look orange or yellow in lower concentrations. If neither pigment is produced, then hair is white. Therefore, all the roans, brindles, speckled patterns, linebacks, grullas, reds, yellows, oranges, browns, and blacks seen in Texas Longhorns come from varying amounts and patterns of expression of these two pigments on different parts of the body. However, the distribution of these two pigments is controlled by a large number of different genes, which makes the inheritance of the two pigments somewhat complex.
All the genes of an organism are together called the genome. To understand the genome of an organism, imagine a language in which all the words are spelled with just four letters. Furthermore, in this simple language, all the words are just three letters long. This is the language of DNA: the four “letters” are called nucleotides, and are typically represented by the letters A, C, G, and T. A gene is simply a long string of these four nucleotides. Each combination of three nucleotides (called a codon) specifies a particular amino acid (the building blocks of proteins). Proteins, in turn, control almost everything about the way an organism is constructed and the way it functions. Proteins are more complex than DNA, but there are still just 20 basic amino acids. Different proteins are simply different strings of these 20 basic building blocks. Every structural gene of an organism codes for a different string of these amino acids. Each string of amino acids makes up a particular protein, and the details of the proteins produced determine virtually all the differences we see among different individuals and species.
The best-studied gene related to color is the gene that controls the relative degree of eumelanin and phaeomelanin production. This gene (known as the Extension locus) regulates the levels of an enzyme called tyrosinase. Low levels of tyrosinase result in phaeomelanin production, and high levels of tyrosinase result in production of eumelanin. The wild-type allele results in variable but typically intermediate amounts of tyrosinase. This means that both eumelanin and phaeomelanin are produced, and the ratio and distribution of the two pigments may be modified by other genes. Longhorns that have two copies (one from each parent) of the wild-type allele at the Extension locus are typically some shade of brown at birth, but often grow darker as they grow older (and may appear black as adults). The relative expression of eumelanin appears to be related to the sex of the animal, and males (with wild type alleles at the Extension locus) are more likely to be black as adults than are females. However, the muzzle ring of these individuals is usually tan or brown rather than pure black. Other individuals with the wild-type allele may be dark brown (including Parker brown), medium brown, or a mixture of brown and black or red and black (including brindling, “wine-colored,” and many of the other unusual colorations of Longhorns). The dark brown pattern (tending to black in males) is thought to represent the ancestral coloration of the wild Aurochs, from which modern Bos taurus cattle breeds have descended.
There are two well-studied alleles (forms of a gene) that differ from the wild-type allele at the Extension locus in cattle. Mutations in DNA can take several forms, including substitutions (replacing one “letter” in the DNA sequence with another) and deletions (removing a letter from the sequence). Substitutions often change the function of a gene, but may not render it functionless. Deletions, on the other hand, often destroy the function of a gene. One of the alleles at the Extension locus differs from the wild-type allele by a single substitution. This simple change results in a single amino acid replacement in the gene, which in turn results in an excess production of eumelanin in skin and hair cells. Because the hairs have an excess of eumelanin, any colored hairs will be black at birth (other genes may keep some of the hairs from expressing any pigment, so the calf is often black and white). Even if a calf inherits just one copy of this gene, almost any hair that is pigmented will be colored black. Thus, if a bull (or cow) has inherited copies of this black allele from both its mother and its father, then all of its own offspring will also express black (no matter what gene is inherited from the other parent). This is how some breeders can guarantee that their bulls will always produce calves with black coloration: the bull has been tested and found to be homozygous for the black allele at the Extension locus (which simply means that both of its copies of this gene are the dominant black allele).
The other common allele at the Extension locus in cattle is a deletion mutation (a single nucleotide has been lost), which results in a non-functional gene. If an animal has two copies of this allele, then that animal does not produce normal tyrosinase and therefore lacks the ability to produce eumelanin. Phaeomelanin is still produced, however, so any pigmented hairs have a basic red coloration. If an animal has only one copy of the red allele and one of the wild-type allele, then there is still enough tyrosinase produced for the coloration to appear just like a homozygous wild-type individual (i.e., brown, but typically darkening with age, especially in males). Thus, the wild-type allele is said to be dominant over the red allele, since an animal with both alleles will show the wild-type coloration. On the other hand, the black allele is dominant over both the wild-type and red alleles, since even one copy will result in an over-abundance of eumelanin. Hence, a cow or bull that is black at birth may be homozygous black, or heterozygous black and wild-type, or heterozygous black and red (any of these combination will simply look black). A calf with wild-type coloration may be homozygous wild-type, or heterozygous wild-type and red. Finally, a true red calf is always homozygous for the red allele.
The “black” allele is abbreviated ED (the E stands for Extension, and the D stands for dominant black), the wild-type allele is abbreviated E+ (the + symbol is used to designate the wild-type allele at any given locus), and the red allele is abbreviated e (lower case is used to indicate that this allele is recessive to the other two alleles). We indicate the dominance order of these three alleles by writing ED > E+ > e. With this shorthand, we can indicate a homozygous black bull by writing that its genotype is ED/ED, whereas as heterozygous wild-type/red bull would be said to have the genotype E+/e.
If these are the three basic colors of cattle, then what produces colors such as grullas, duns, light reds or oranges, and yellows? There are at least two additional loci that can reduce the total amount of pigment produced in a given hair. One of these genes is known as the Dilution locus. The common (wild-type) allele at this locus in Texas Longhorns (ds+) produces no change in the coloration (so a black cow will appear black, if it is also homozygous ds+/ds+). However, if a cow inherits one copy of an incompletely dominant dilution allele (Ds), then a black cow will appear gray, a brown cow will appear lighter brown, and a red cow will appear light red or orange. If both copies of the gene are dilution alleles (i.e., the animal is homozygous Ds/Ds), then the black cow will appear light gray (grulla), the brown cow will be a dun, and the red cow will appear yellow. However, all duns do not result from the dilution locus. Allelic variation at another locus (called Dun) can also produce the dun coloration. The allele dn is incompletely recessive to the wild-type allele at this locus (Dn+). Individuals that are heterozygous Dn+/dn have a reduced amount of red pigment (phaeomelanin), but there is little effect on black pigment (eumelanin). If a cow or bull has the ED (black) allele at the Extension locus and is also heterozygous Dn+/dn at the Dun locus, then it will likely have some dark brown (rather than black) along the black and on the poll. The effect on red is more uniform, and homozygous red (e/e) animals that are Dn+/dn will be duns (the color is even lighter in dn/dn animals).
Brindle coloration always occurs in cattle that are either homozygous wild-type (E+/E+) or heterozygous wild-type/red (E+/e) at the Extension locus. In addition, they must also have at least one copy of a dominant allele at the Brindle locus (Br). Although the combination of E+ at Extension and Br at Brindle will always produce some kind of brindle pattern, this pattern usually takes several years to develop. Even then, the brindle coloration will only appear in areas that would have exhibited the wild-type brown coloration in the absence of the Br allele. Thus, brindle patterns interact in a wide variety of ways with other genotypes, including the various white-producing patterns (see below), as well as with the Dilution and Dun genes. For an example of one of these complex brindle patterns, see CO Barbwire.
Additional genetic loci produce various amounts of white coloration in Texas Longhorns (and other cattle). The loci that produce areas of white coloration (actually lack of pigmentation) in Texas Longhorns include the Spotting, Roan, Brockling, and Color-sided loci.
Spotting (SP > S+ > s)
The wild-type allele (S+) at this locus produces no white. However, at least two other alleles at this locus are found in Texas Longhorns, each of which produces areas of white on the coat. One (SP; the P stands for Pinzgauer) is incompletely dominant to the wild-type allele. If the SP allele is present, it produces the line-back pattern (e.g., see TP Bo Peep or her calf, D-H Oreo). However, Texas Longhorns that are heterozygous SP/S+ exhibit smaller areas of white coloration than do individuals that are homozygous SP/SP. (TP Bo Peep is heterozygous SP/S+ and D-H Oreo is homozygous SP/SP). Another allele at this locus is the recessive s allele, which (when homozygous) produces large areas of white coloration that can vary considerably in size from one individual to the next (probably due to additional modifying genes). D-H Firecracker is an example of a cow that is homozygous s/s at the Spotting locus and heterozygous wild-type (E+/e) at the Extension locus (and heterozygous for the brindle allele, but the brindle is just beginning to develop). Overlord CP and D-H Shogun are examples of bulls that are homozygous s/s at the Spotting locus and are heterozygous black at the Extension locus (all of these animals also carry the brockling allele; see below).
Roan (R > r+)
The incompletely dominant R allele at the Roan locus restricts pigmentation (both phaeomelanin and eumelanin). Texas Longhorns that are homozygous red (e/e at the Extension locus) and heterozygous R/r+ at the Roan locus will be red roans (a mixture of red and white hairs will be produced). Likewise, cattle that have at least one copy of the dominant black (ED) allele at the Extension locus and are heterozygous R/r+ at the Roan locus will be blue roans (a mixture of black and white hairs). Homozygosity for the R allele produces an almost completely white Texas Longhorn (with pigmented ears), no matter which alleles are present at the Extension locus.
Brockling (Bc > bc+)
The dominant brockling allele (Bc) interacts with other loci to produce pigmentation in areas that would otherwise be white. The most common type seen in Texas Longhorns is in animals that are homozygous s/s at the Spotting locus. If the animal carries at least one allele of Bc as well, then the legs will be pigmented, whereas if the animal is homozygous bc+/bc+, then the legs will be white. D-H Firecracker is an example of a cow that is homozygous s/s at the Spotting locus and carries at least one copy of Bc at the Brockling locus.
Color-Sided (Cs > cs+)
The color-sided phenotype is produced by heterozygosity of a partially dominant allele at the color-sided locus (i.e., individuals with a Cs/cs+ genotype). Color-sided cattle typically have a very irregular white stripe (with roan or dappled edges) along the backs and belly, and a roan or dappled pattern on the head. However, the expression is somewhat variable, and some heterozygotes for Cs show much less white on the back and belly, and little to no white on the head. In cattle that have at least one copy of the ED (black) allele at the Extension locus, heterozygotes for Cs/cs+ may even appear to be blue roans. Cattle that are homozygous for the Cs allele (i.e., Cs/Cs) exhibit the “White Park” pattern: a mostly white animal with areas of color on the ears, muzzle, and lower legs. However, this is not the only genetic locus that produces this phenotype. For instance, L Brilliant Mary exhibits the White-Park pattern, but is not homozygous for the Cs allele (see more below). The color-sided pattern is somewhat similar to but has a different genetic basis than the lineback or Pinzgauer pattern (see above). To see the difference between the true lineback phenotype and the color-sided phenotype, compare TP Bo Peep (lineback) and CO Paint Brush (color-sided) on our breeding stock page. You'll notice that the edges of the pigmented areas are sharper in the lineback pattern, and also notice that the color-sided pattern involves a roan or dappled pattern on the head.
Interactions Among the White-producing Loci
There also appear to be additive effects across many of these white-producing loci. For instance, individuals that are heterozygous for Cs at the Color-Sided locus and also heterozygous for R at the Roan locus will express the White-Park phenotype (or a slight pattern variant that is common in Texas Longhorns of the Butler family, called “flea-bitten,” which usually shows some small spots of color on the body, as well as the head and legs). Similarly, heterozygotes for Cs and SP will be almost completely white (with even less coloration than in the White-Park or flea-bitten phenotypes). Therefore, it is often not possible to predict the genotype of a given animal by its color alone (especially if it is mostly white), without also examining its ancestors and/or offspring. For instance, consider the coloration of L Brilliant Mary, who shows a classic White-Park type pattern. Her sire, Monarch 103, was a product of Bevo and Lady Butler. Lady Butler, in turn, was a product of Bevo and Beauty, two of the most famous Texas Longhorns from the Butler family. Both Bevo and Beauty were heterozygous for Cs at the Color-Sided locus, and their daughter Lady Butler appears to have been homozygous at this locus, as was Lady Butler's son Monarch 103. So, we can assume that L Brilliant Mary inherited at least one Cs allele at the Color-Sided locus from her sire. Her White-Park pattern could then be a result of having two copies of the Cs allele at the Color-Sided locus, or it could be from an interaction between a heterozygous Cs/cs+ at the Color-Sided locus as well as heterozygosity at the Roan and/or Spotting loci. To determine which of these possibilities applies, we can look at her offspring. Her 2003 calf, D-H Rising Sun, was a result of a cross with Wind Chill, a mostly solid wild-type colored bull with some scattered white spotting, including white markings on the face. Since D-H Rising Sun also has solid wild-type coloration (a deep, rich red at birth) with just a small white marking on the head, we know that L Brilliant Mary cannot be homozygous for the Cs allele at the Color-Sided locus (if she were, her calves could be color-sided or White-Park, but not solid). Therefore, her White-Park pattern is the result of an interaction of heterozygous loci at the Color-Sided locus and either the Roan or Spotting locus. Her 2004 calf, D-H Kickapoo, was the result of a breeding with a spotted and brockled black-and-white bull, D-H Shogun, and the calf shows a similar color pattern to the bull. Therefore, we can tell that L Brilliant Mary is heterozygous at the Spotted locus, since her 2004 calf is homozygous for the recessive s allele at the Spotted locus. This means that despite her mostly white coloration, she can produce a very wide diversity of color patterns in her offspring, including solid, color-sided, and spotted. In addition, because she is homozygous for the recessive e allele at the Extension locus, she can produce red, wild-type (including brindle and other variants), or black (including grulla) calves, depending on which bulls are used with her. I sometimes see breeders who avoid mostly white cows because they are afraid that they will produce mostly white offspring. However, as this example shows, this is not necessarily true, because mostly white cows can be produced from many different combinations of genes. In some cases (as with L Brilliant Mary), mostly white cattle can produce a very wide diversity of colors and patterns in their offspring. So, for the breeder who likes color and pattern diversity, these cows can be highly desirable. Often, it is possible to determine the genotype of such cows by examining their ancestors and previous offspring.
Cows like L Brilliant Mary often lead breeders to say that “you can't breed for color in Texas Longhorns,” because these animals produce such a wide diversity of offspring. It is true that the genetics of coloration is not simple in Texas Longhorns, because this breed contains much of the diversity for coloration that is known among all breeds of cattle. There are many genes that affect coloration, and these genes often interact with one another in interesting ways. However, it is possible to choose or breed cattle to produce particular color patterns, if the breeder simply understands the genetic loci that are involved. For instance, it is possible to produce an all-brindled herd by fixing the wild-type allele at the Extension locus and the Br allele at the Brindle locus, and eliminating all the white-producing alleles at the other loci. This can be done by selective breeding and culling. Some breeders who raise Texas Longhorn bulls for use on commercial beef herds have eliminated all the white-producing loci from their herds, to produce all solid bulls (red, brown, and black). If one wants to produce all grulla calves, a bull that is homozygous for Ds at the Dilution locus and also homozygous ED (black) allele at the Extension locus (and is homozygous for the wild-type alleles at the various white-producing loci) will produce grulla offspring with any cow (darker grullas with cows who have no Ds alleles, and lighter grullas with cows that also carry the Ds allele). For breeders who simply want lots of color and pattern diversity, and enjoy being surprised by each new calf, bulls and cows that are heterozygous at many of the color-pattern loci are the best choice. For instance, the cross of a bull and a cow that are both heterozygous ED/E+ at the Extension locus, heterozygous Br/br+ at the Brindle locus, heterozygous SP/S+ at the Spotting locus, heterozygous Cs/cs+ at the Color-sided locus, heterozygous R/r+ at the Roan locus, and heterozygous Ds/ds+ at the Dilution locus could produce Texas Longhorns of virtually any of the major colors and patterns that occur in the breed, and very few of their calves would look alike (this bull and cow could produce calves of 729 different possible genotypes for just these six color and pattern genes). And yet, a cow and bull of this genotype would both be almost completely white, with only their ears showing a little gray coloration!
Considering all eight genetic loci (the ones discussed in this article) that are known to affect color and pattern in Texas Longhorns (Extension, Brindle, Dilution, Dun, Spotting, Color-sided, Roan, and Brockling), there are 26,244 different possible genotypic combinations of known alleles that can appear in an individual bull or cow. In addition, there are almost certainly many additional genes that affect color and pattern that have yet to be described, so the number of possible genotypic combinations for color and pattern is almost certainly many times larger. If we assume just two additional genes, each with three different possible alleles, then the number of possible genotypic combinations increases to 944,784! Since this latter number exceeds the number of registered Texas Longhorns, then there are almostly certainly possible genotypic combinations of color/pattern alleles that have not yet been observed in modern Texas Longhorn herds. Of course, all of these genotypic combinations do not produce distinctly different phenotypes, but this helps to explain why Texas Longhorns are “more varied than the colors of the rainbow,” as J. Frank Dobie wrote in The Longhorns.
For examples of many of the color patterns discussed on this page, see the discussions of genotypes (under the pedigrees) on our breeding stock pages. Just click on the individual cows or bulls to read about the genotypic basic of their coloration.
Finally, I should emphasize that what I've written here represents our present knowledge of Texas Longhorn coloration...but there is still much to be discovered. Additional genes almost certainly influence the coloration of cattle, and there are probably many genes that have a modifying influence on the principal genes discussed here (which is one reason why no two Texas Longhorns look exactly alike). If you find exceptions to the genetic patterns discussed on this page, I would be happy to learn about them. It is from the exceptions that we can learn about new genes and alleles that influence the coloration of Texas Longhorns.
Want more detail? I have expanded this page into a five-part series of articles on the Genetics of Coloration in Texas Longhorns for the Texas Longhorn Trails. The first of these articles appeared in the June 2004 issue of the Trails, and they will continue through November 2004. There are links to these papers below.—DMH.
Additional Reading and References
Joerg, H. et al. 1996. Red coat color in Holstein cattle is associated with a deletion in the MSHR gene. Mammalian Genome 7: 317-318.
Klungland, H. et al. 1995. The role of melanocyte-stimulating hormone (MSH) receptor in bovine coat color determination. Mammalian Genome 6: 636-639.
Lauvergne, J. J. 1966. Génétique de la couleur de pelage dex boivins domestiques. Bibliographia Genetica 20:1-168.
Olson, T. A. 1980. Choice of a wild-type standard in color genetics of domestic cattle. Journal of Heredity 71:442-444.
Olson, T. A. 1981. The genetic basis for piebald patterns in cattle. Journal of Heredity 72:113-116.
Olson, T. A. 1999. Genetics of Colour Variation. In The Genetics of Cattle (R. Fries and A. Ruvinsky, eds.). Pp. 33-53. CABI Publishing, Wallingford, United Kingdom.
Robbins, L. S. et al. 1993. Pigmentation phenotypes of variant extension locus alleles result from point mutations that alter MSH receptor function. Cell 72:827-834.
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