Suwannee Cockers
The Place for Natural Merles and Sables
COAT COLOR AND MARKINGS GENETICS IN AMERICAN COCKERS
The first thing that most people usually notice about a dog is its color, yet not a lot is really known about the genetics involved in the inheritance of coat color and markings in dogs. And what is known usually comes from observations of long time breeders, and a few books that have been written over the years about the subject, which unfortunately contain a lot of speculation. However, there is now quite a bit of serious genetic research with dogs in university settings, and we hope to soon know much more about the inheritance of coat color and markings in dogs, and especially in particular breeds.
Considering the complexity and the "unknowns" in the subject, this discussion is limited to what is commonly thought to be true in color genetics with American Cockers, with just a few comments about other breeds. And this is not a "scientific" discussion, with lots of footnotes and references, but just a general overview of the subject, written as if the reader knew absolutely nothing about dog color genetics, as when we first started trying to learn about color genetics in dogs, we often didn't know enough basics in the subject to understand what we were reading. And for those of you who would like to pursue this subject further, we have provided links to other websites devoted to dog color and markings genetics, listed on the left side of this page. We've chosen these particular websites either because they have a lot of pictures, or have very informative charts, or the breed discussed has similar colors and markings as Cockers, or the colors in the breed are just so strange and unexplainable with our current knowledge of dog color genetics--see the Coton, Bearded Collie, Lowchen, and Dalmation sites--that it boggles the mind. But we are always looking for more websites to add to this list. We also appreciate being notified of any dead links.
We do know that dog coat color and markings genetics is basically the same as human genetics, centered in the genes and chromosomes, but it might help to try to visualize all of this talk about genetics as a lot like Algebra, in which we use symbols to stand for the unknown, helping us to visualize the structure, but the structure is still vague in the details!
Please note that in this discussion we have placed a lot of pictures of dogs throughout the text, first to break it up, so that it is easier to read, and also to somewhat illustrate what is written in the text, and if you will hover your cursor above each picture, a caption for that picture will appear. Also please note that the writer is not a scientist or a geneticist, but is a long-time dog breeder with an abiding interest in coat color and markings genetics, so the following is simply the writer's current understanding about this subject, and this discussion is in no way the definitive word on dog coat color and markings genetics!
And now some basic genetic terms: a chromosome is a linear body contained within the cell nuclei of plants and animals which contains DNA, and the chromosome is responsible for the determination and the transmission of hereditary characteristics. All of the cells in our bodies contain chromosomes in their nuclei, and the chromosomes are continually duplicated through cell division in our bodies, with the DNA providing a "blueprint" for how each new cell is to be formed. But the reproductive cells have special properties in that they can split their chromosomes in half (meiosis) when producing the egg and the sperm, so that when the egg and sperm meet during reproduction, the new puppy that is created has half of its genetic inheritance from its mother (dam) and half of its genetic inheritance from its father (sire), and the puppy is now a combination of chromosomes from each parent.
However, during a stage in meiosis, just before the chromosomes in the "parent-to-be" divide themselves, the long strands of DNA within the chromosome line up together and actually exchange segments, so by the time of actual separation, each of the resulting chromosomes that will be "thrown" to the next generation contain a mixture of genetic material that the reproducing dog inherited from its own two parents. In this way bits and pieces of genetic material are being continually mixed in each dog, so there is really no "unbroken" lines of inheritance from generation to generation through only the dam's or the sire's lines, except perhaps in mitochondrial DNA, which we are not discussing here. And thus each puppy is a result of thousands--perhaps more--of bits and pieces of genetic material from the past, making up one unique indidividual.
Breeders often "line or in-breed" in order to "fix" type and/or color in their dogs, hoping to eliminate surprises by limiting their gene pool, but this method of breeding often also establishes stubborn health problems within such a limited gene pool. As breeders of pets with non-standard colors we do feel freer to out-breed than say a show breeder, using temperament as our guide more than breed type, or even color, thus opening up our gene pool to more genetic variation. The following is a quote from a genetic population article that we found on the National Biological Information Infrastructure (NBII) Genetic Diversity website (see the links under "About Color Genetics" to the left of this page) and we discuss heterozygous and homozygous genes later in this article:
One of the effects of inbreeding is a decrease in the heterozygosity (increase in homozygosity) of the population as a whole, which means a decrease in the number of heterozygous genes in the individuals. This effect places individuals and the population at a greater risk from homozygous recessive diseases that result from inheriting a copy of the same recessive allele from both parents. The impact of accumulating deleterious homozygous traits is called inbreeding depression - the loss in population vigor due to loss in genetic variability or genetic options.
So by mixing up our colors and markings we are thus opening up our gene pool to more variety within the breed, and we do feel that this is a "good thing" for our puppies. And since Spaniels have been pretty much "purebred" for at least 700 years now, its not as if we are mixing established breeds, as in Cock-A-Poos. But we do not feel that there is anything wrong with all of the new mixes, such as all of the various "Poo" and "Doodle" dogs, as we feel that people should be free to purchase any type of dog that they desire, as long as it is basically healthy and makes a good pet. And studying the history of dogs has made us realize that dogs are constantly evolving and changing, if only from natural mutations and genetic "drift."
But by our outbreeding so much we certainly do get our share of "surprises" along the way in our breeding program, and our male named SunCatcher's Zaffre Zecchino, shown just below at about six days old, is an example of a complete surprise to us, as he is a Sable-Merle with blue eyes but with a coat marked more like a Calico Cat, and we are still scratching our heads as to where this marking came from.
But back to the chromosomes: in each species each chromosome holds a certain number of genes, and in order to produce living offspring, each member of a species must have the same number of chromosomes and kinds of genes to meet during fertilization and produce offspring. Each dog has 39 pairs of chromosomes, and each chromosome contains thousands of genes.
However, it is possible to breed two closely related species, each with a different number of genes, like female horses bred to male donkeys, and produce another type of animal like mules, but mules are almost always sterile, so cannot be bred together to produce more mules, and are thus a sterile hybrid and not an ongoing species. Stallions--male horses--can also be bred to female donkeys to produce hinnies, but mules are usually preferred as a working animal over hinnies. But generally animals that can be bred together are of the same species, with the same number and type of genes, and dogs and wolves are still of the same species, because they can still interbreed and produce viable offspring, which can in turn reproduce themselves. So the whole "wolf-hybrid" issue is kind of silly, as hybrids are a result of crosses between two different species, and are generally infertile.
When the same two types of genes meet in a puppy they have a place, or "locus"--a Latin word meaning "place"-- waiting for them on the chromosome, and in genetics these are called "receptors." And the gene is what we are trying to understand here, as it is the functional hereditary unit, occupying a fixed position on a chromosome, and having a specific influence on the phenotype--what the dogs looks like--and the gene is quite capable of undergoing mutation to various allelic forms. And by calling a very specific gene an allele, we are showing that there can be many variations of the same gene at that locus. For example, at the human eye color locus, we can have blue, green, and brown alleles, or genes, and these eye colors follow genetic inheritance laws, just like dog coat colors and markings.
An example of two different alleles of the same gene are black and brown dogs, which are the only two different alleles possible on the B Locus in dogs. It is believed that black was the original color of all dogs and that brown is a later mutation. Brown colored dogs are most often found in the sporting or hunting breeds, and it is thought that the brown mutation was selected and bred for by hunters because the brown color affords more camouflage out in the field than the color black.
The cells of the body most involved with coat color are the melanocytes, which are skin cells capable of synthesizing the pigment melanin, and the melanocytes are responsible for color variations in the skin of humans and many other animals. Melanocytes are responsible for tanning in humans, producing the colors in a dog's nose leather, and giving a dog's hair its color. There are two distinct types of melanin in the dog, as well as in many other mammals, one called eumelanin, which produces black or dark brown pigment, and one called phaeomelanin, which produces a yellowish color.
In the developing embryo the pigmentation pathways are shared with other pathways, so any mutation in the genes affecting coat color can also affect neurological, immunological, and other pathways in the animal, and at early stages of embryo development mutations may cause miscarriages. Also many breeders believe that differently colored dogs within a breed have distinctly different temperaments, according to their color, and there may be some truth to that belief based on genetics. However, as speculation about this subject often leads to prejudice against certain colors, we will go no further with this particular subject.
In the developing pup embryo, the melanocyes migrate down from the "neural crest derived cells" along the spinal column and brain, and the last places the melanocyes migrate to are the chest and the feet, so many dogs don't have pigment in these areas, as something as simple as a cold for a few days in the mother dog during gestation can interfere with this migration, leaving these areas with no pigment to speak of. Also it is known that red dogs, across many breeds, have a thirty percent more chance of having unpigmented, or white areas, than other colors, but the reason for this is unknown at this time. Also certain mutations can cause the loss of melanin altogether, and as the development of both the eyes and the ears depend on melanin to develop properly, an all white dog in a breed that isn't usually all white will often be born blind, deaf, or both, or perhaps without any eyes at all.
But back to the migrating cells: in the beginning of this migration the melanocytes actually don't exist--the migrating cells involved are evolving into both neural precursor cells and pigment precursor cells, and mutations at this time can prove fatal. Other less lethal mutations early in this stage cause the domestic pig to be white--really just one big white spot--and "roan" cattle. Mast cells are involved here, and these cells are also important in the developing immune system.
ALL DOGS ARE BLACK
Underneath all of the different coat colors and markings in dogs, all dogs are black, and all of the other colors and markings are produced by alleles of genes that modify this basic black color. In the wild the basic black coat may be solid, spotted or ticked, but humans have been around domesticated dogs for such a long time--over 100,000 years--that there has been a lot of time for breeders to select for the various mutations that showed up in puppies over time, and to perpetuate these mutations through selective breeding, so that we now have hundreds, if not thousands, of different breeds of dogs, in all sizes, shapes and colors. However, most of the breeds that are available today have only been developed during the past 500 years, by mixing more ancient breeds, and many of these ancient breeds are now lost to us, and it is only through the continuing dedication and perseverance of the "pure bred" breeder that the living breeds have been preserved up to the present time.
There are various systems used for representing coat color genes in dogs, but we are mostly going to stick to the old tried and true alphabetic letters used by Clarence Little in his book, "The Inheritance of Coat Color in Dogs," written in the 1950's. It is no doubt oversimplified, but has the advantage of taking a complex idea and breaking it down into managable sections--like Algebra--and the interaction of the genes in his system can be plotted out simply and rather accurately with Punnett Squares, those handy little diagrams Cocker breeders can be found scribbling on the backs of envelopes while anxiously awaiting the birth of a litter.
But before we get into our discussion of the specific genes, we need to understand how color and markings genes work in the dog, which requires understanding certain genetic terms like "dominant" and "recessive" alleles, "epistasis" between genes, and what the terms "homozygous" and "heterozygous" mean. We also need to understand the various types of genes, according to their function. For instance, in Cockers the S locus controls the distribution of color on a dog, while the A, B, C, D, and E loci control the color of individual hairs. And there are "specialty" genes like the M, R, and T loci that produce certain specific types of markings and/or colors.
Dominant and Recessive: When we say that an allele of a gene is "dominant" we mean that if it is present it will express itself, hiding any other recessive alleles of that gene, and thus one can always see dominant alleles. This means that if the two alleles--one from each parent--that a puppy inherited at a specific locus are different, then the dominant allele will determine what the dog looks like, and this is called a heterozygous dog--which means that it has two different alleles--on that gene locus. Thus recessive traits will only show if BOTH alleles the puppy inherited at a gene locus are that particular recesive allele, making the puppy homozygous--or having alike alleles--for that particular gene locus.
So when one is speaking of a "dominant gene" one is really speaking about the dominant allele, out of several alleles, on just one gene. An example of a dominant allele in American Cockers is the color black, which if present on the B Locus, will show on the dog, as long as there are no other epistatic alleles from other genes--like Buff--to hide it. An example of a recessive allele is the color brown--usually called chocolate in Cockers--which the puppy must inherit from both parents on the B Locus before the color brown can be seen on the pup.
A dog can inherit one black allele from one parent and one brown allele from the other parent, but since black is dominant this dog will be black, but carrying brown recessively, and this dog can have brown offspring if bred to another dog that is itself brown, or also carrying brown recessively. In a breeding of a black dog carrying brown recessively, bred to a brown dog, half of the puppies will be black and half will be brown, but if both dogs are black carrying brown recessively, then three quarters of the puppies will be black and one quarter will be brown. But these are statistics that will bear out over 100 puppies born to any two dogs, and thus these genetic statistics do not necessarily show up in any particular litter, and can only be used as a guide as to what colors are possible in any breeding.
The two dogs pictured here at the left are our chocolate (brown) Tricolor (tan pointed) Roan girl named Suzy Q, and our now retired black based Blue Merle and white boy named ZuZu. We believe that ZuZu's silvery Buff dam is either a chocolate based dog herself, or carrying chocolate recessively, as she threw some chocolate pups, one of which was in ZuZu's litter. ZuZu's dam has a rather brown looking nose and eyerims, so we think that she is a brown based buff dog, but her two litters only produced two brown pups, and a large majority of her pups were black, as she was bred to our Max, ZuZu's sire, who we believe is a black carrying brown recessively. So if ZuZu's two parents are both black based, but both carrying brown recessively, then ZuZu had a 50% of carrying chocolate, which indeed he did, as bred to our Suzy Q for one litter they produced one chocolate tri puppy, and chocolate is recessive so it has to come from both parents. However, both ZuZu and Suzy Q proved to be carrying buff recessively too, and three of their five puppies were buff and whites. One puppy was the chocolate tri, and the fifth puppy was a black and white, so this litter is a perfect example of dominant/recessive traits at work.
We often have to work backward in dog genetics, and the above litter proves several things about what both Suzy Q and ZuZu are carring in their color genetics. First it proves that both dogs are carrying buff recessively on the E Locus, because over half of the litter is buff and white. Statistically half of the litter should be buffs if both dogs are carrying buff recessively, so three out of five is just a bit more statistically. But the buff and white puppies could be either black based or chocolate based, and that is often difficult to determine in a young puppy. Of course all of the pups inherited chocolate from their dam, Suzy Q, but in order for there to be the one chocolate puppy, ZuZu also has to be carrying chocolate recessively, as we suspected, so some of the buff and white puppies could also be chocolate based. The litter tells us other things also, such as ZuZu is a dominant black on the K Locus--which we weren't really sure of, as Merle can scramble both Sable and tri into the Merle coat. But Dominant black is epistatic, or hides both Sable and Tris, so the one black and white puppy shows that ZuZu is a hererozygous Dominant Black on the K Locus.
ZuZu also has to be heterozygous "K k" on the Dominant Black K Locus, because he threw the one tri puppy, because he is not a tri, but that tri puppy has to be "k k" on the K Locus in order for the tri to show, so one of those "little k's" had to come from ZuZu. Also tri is recessive on the A Locus, thus it takes a tri allele from both parents to create a tri puppy, so this one chocolate tri puppy also tells us that ZuZu is carrying tri--"at at"--on the A Locus, and probably not carrying Shaded Sable, although it's possible. Roan is also dominant on the R Locus, and before they left here almost all of these puppies showed some type of heavy ticking, or roaning, as can be seen on the legs of the chocolate tri pup at nine weeks old, so probably none of these puppies wound up having "clear" white areas, without ticking or roaning. Now if you haven't followed all of this here, we will be going into all of these genetic coat color and markings terms later on in this article. Have patience! But this litter is certainly a good example of the action of both dominant and recessive alleles on a gene, with the buff being carried recessively by both dogs, and the chocolate being carried recessively by ZuZu.
But if colors in dogs were a simple dominant/recessive action, then we wouldn't have any problems understanding dog coat color and markings genetics. But some color and markings genes are arranged in "tiers," so that one set of alleles has to be present in the dog before another allele on a separate gene can take any action, as some colors and/or markings are simply not possible unless particular alleles occur at more than one locus. For instance a brown dog not only has to have two recessive alleles on the B Locus--called "b b" on the B Locus--but it must also have at least one dominant "E" allele on the E Locus, in order to be a brown dog, as dogs having only recessive "e e" on the E Locus are a buff dog such as Darlin, the buff dam of ZuZu, pictured above. In this picture Darlin's nose looks black, but Darlin changes colors quite frequently, and sometimes she exhibits a lighter "Winter Nose," which looks quite brown.
Epistasis: And in the newest genetic scheme that we have been reading about, which changes "dominant black" from the most dominant allele on the A Locus (according to Little's genetic schemes) to the most dominant allele on a newly postulated K Locus, recessive "e e" (Buff) on the E Locus is eipstatic, when present, to the newly located K Locus (dominant black), meaning that Buff can actually hide dominant black. And in turn the K Locus is epistatic to the A Locus, meaning that dominant black can hide Sable and Tri. This picture is an example of an A Locus Sable and white puppy with his black base coat color diluted to a mahogany red color--Sable is a dilution gene--leaving black Shaded Sable markings on the tip of his ears and on his back and sides, with part of a dark mask showing on the sides of his muzzle. His Shaded Sable coloring is caused by the Sable alleles "ay at" on the A Locus, and his Sable color is not hidden by either dominant black on the K Locus--either homozygous "K K" or heterozygous "K k"--or by Buff (recessive yellow)--"e e"--on the E Locus, so his Sable dilute coloring shows through. This genetic scheme certainly would explain why Sables are so rare, as Sables cannot make their appearance except in the absence of two other genes which hide Sable by epistasis when either gene is present. This picture is of our Sable and white, Cryptic Merle, male named Max at about four months old.
Homozygous and Heterogygous: When a dog has inherited two black alleles for the B Locus gene, we say that this dog is "homozygous" for black--"B B"--and this dog can only throw the color black. But if a dog has inherited one black allele and one brown allele for the B Locus gene--"B b"--then we say that this dog is "heterozygous" for the B Locus gene, and this heterozygous dog can produce both black and brown pups, depending on the genetic makeup of its mate, as we discussed above. So in this way recessive alleles can be carried unseen under dominant alleles for generations, thus often surprising the breeder down the line. Tan Points are a good example of a homozygous gene, as Tan Points are the most recessive allele on the A Locus in Cockers, and in order to show Tan Points a dog must be homozygous for the Tan Pointed allele, like the Black and Tan puppy shown here named Pippin, who is out of our Max and Sparkle, and lives with his family in Titusville, Florida. But like Sable, Tan Points won't show if the dog is either a Buff on the E Locus or a Dominant Black on the K Locus.
We have a friend whose Buff girl recently produced one Tan Pointed puppy in a litter, after having several litters with no Tan Pointed puppies at all, and this dam had no Tan Points showing in her pedigree for ten generations! Of course there were probably Tan Pointed dogs in some of the littermates of the dogs in the pedigree, but unless the breeder was familiar with all of the dogs mentioned in the pedigree, and their littermates, then the pedigree itself would not reveal that Tan Points are possible. Tan Points are the most recessive allele on the A Locus in Cockers, and both parents have to throw the Tan Points allele for the puppy to show them. But despite such surprises, pedigree research will really pay off in predicting puppy coat color and markings, providing that the breeders in the past were both knowledgable about the colors that they were observing and breeding, and that they were also honest in recording the colors correctly.
Our now retired red girl named Sparkle, to the left, is a solid red with white markings like Tan Points, and her sire is Tan Pointed, and she sometimes throws Tan Pointed puppies, like Pippin, but we are not sure if she is a solid dog carrying one tan point recessively, making her heterozygous on the A Locus, and thus a hidden Shaded Sable, which is hidden by Dominant Black on the K Locus; or if she is herself a Tan Pointed dog, with two Tan Pointed alleles, but with the Tan Points diluted to white, making her homozygous for Tan Points on the A Locus. The only way to tell if Sparkle is homozygous or heterozygous on the A Locus would be to breed her to a Tan Pointed dog and then see if all of her puppies turned out to have Tan Points, as Tan Points bred to Tan Points would be two homogygous alleles bred together, and being recessive, Tan Points cannot carry any other more dominant alleles, so only Tan Pointed puppies would be possible from such a breeding.
However, some dilution alleles do not act on the puppy immediately, but show themselves at a later age--sometimes as late as a year old--so sometimes even the most scrupulous of breeders will get colors wrong on the registration papers for the dog. Our retired red girl named Sparkle, is also an example of this, as when we registered her with the AKC as an 8 week old pup she appeared to have enough white areas to be called a red and white, but as she grew up some of her white areas turned to "blonde," and she actually turned out to be a solid red dog with a few white markings, but with quite a bit of lighter "blonde" feathering. The three colors are confusing, and even our vet calls Sparkle our "red tri." Technically a dog must have at least 10% of white on its body to be called a parti--or a red and white, as listed on Sparkle's AKC registration papers--and Sparkle appeared to have over 10% white areas when she was a puppy, thus we listed her as a red and white on her papers. But Sparkle throws one-half solid pups when bred to a parti, so obviouisly her colors are listed incorrectly on her registration papers--but it was an "honest" mistake!
In some breeds the colors have been bred so precisely that there are only homozygous genes left in the breed, and there are no other coat color and marking possibilities except through mutation. So when a breeder says that a certain color does not "exist" in their breed, they are really saying that they believe that all living examples of their breed are homozygous for a certain allele on a gene, and not that the gene in question does not exist in their breed. All dogs--and wolves--have EXACTLY the same genes, or they would not be able to breed together and produce offspring. But all of the different breed characteristics in dogs are caused by the different alleles, or "flavors" of the same genes that exist in the different breeds. We'll go into this again later when discussing the Merle gene.
Perhaps all of this will become clearer when we discuss each of the various genes that control coat color and markings in dogs. But this discussion is further complicated in that Cockers have a lot of different colors, even some "outlaw" colors that are popular and active in the gene pool--like Sable and Merle--so there are probably many alleles, or variations of each gene at play in Cockers, and probably some undiscovered alleles that we really know nothing about yet. So American Cocker color and markings genetics is much more complicated than in the dog breeds that are all one color, or have limited color combinations.
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