The control of coat color and pattern in mammals is complex. Much of what we have learned is from breeding experiments with mice and, to a lesser extent, domestic animals including the dog. The biosynthetic pathways involved in the synthesis of the pigments, and the genes involved in the development of the pigment-forming cells (the melanocytes), the hair follicle and the hair shaft, appear to be very similar in most species. Prior to the advent of modern gene-sequencing techniques, geneticists used this basic similarity to extrapolate from one breed or even one species to another. However, as there are a large number of genes (over 85 have been implicated in the mouse), one often encounters mutations in different genes producing nearly identical phenotypes. Now that some of these genes are being cloned and sequenced, the opportunity is before us to settle some old controversies (while undoubtedly creating some new ones.)
The neural crestPigmentation in mammals is primarily due to the presence of melanin, which is synthesized in specialized cells called melanocytes. Melanocytes come from a population of cells, called the neural crest, that is located on the dorsal mid-line of the early embryo.
Neural crest cells contribute to a wide variety of tissues and organs and have to be "told" what their fate is. If the ones that would normally give rise to melanocytes get the wrong signal, or interpret it incorrectly due to a mutation, they may do something else (or nothing at all). The result would be an animal without pigmentation in the skin or hair. However, mutations affecting these signalling processes generally more than just the melanocytes resulting in various defects. To my knowledge, there are no known mutations of this type in the poodle.
There are also genes that affect the pathways of migration of the cells destined to form melanocytes. This can result unpigmented (white) areas. Two such genes that are found in many breeds are Spotting (S) and Merle (M). The S alleles include:
Dogs homozygous for Irish spotting have irregular white patches. The number and size of these
patches is extremely variable. This is probably the allele that produces mismarks.
The extreme white piebald allele is thought to be responsible for all-white animals in some breeds, but not in the poodle. Mutant alleles of the Merle gene are also rare or nonexistant in the poodle.
Synthesis of melanin (the C gene)There are two related types of melanin sythesized by the melanocytes, dark eumelanin (black or brown) and light phaeomelanin (yellow, tan or reddish). In the hair, melanin is found in minute pigment granules. Various genes affect the number, shape, arrangement or position of these granules, or the type of melanin they contain.
Though there are two main types of melanin, both depend on the enzyme tyrosinase. One thing almost universally agreed upon by geneticists is that true albinos, lacking all melanin-based pigments, result from a deficiency in this enzyme. Albinos (cc) are homozygous for a recessive mutant allele. CC or Cc dogs have full color, as determined by the other genes carried. Albinos have no pigment in the nose, eyes, hair or skin - and are very rare.
In many mammals, there is a third allele, chinchilla (cch ). In mice, this allele produces defective tyrosinase which cannot synthesize the normal amounts of melanin. For some reason, the melanin that is made is primarily the dark eumelanin. The degree to which the coat is lightened depends on the species. The eyes and nose generally remain dark.
In dogs, most authorities classify a chinchilla-like mutation as an allele in the C series, but I have seen no studies establishing that it directly affects the activity of tyrosinase. Chinchilla is said to have no noticeable effect on eumelanin, but reduces the color to cream in dogs that would otherwise be tan, apricot or yellow (golden). If this is correct, then a black or brown poodle should be unaffected, but a "chinchilla-apricot" (cchcchee) would be cream.
The two melanins (and the B gene)Tyrosinase converts tyrosine to a compound called DOPAquinone, which leads to phaeomelanin. The genes controlling the remaining steps have not been clearly established, but phaeomelanin appears to be the "default" pigment, and in the absence of other necessary enzymes and regulatory factors, you will end up with an apricot poodle. In some cases modification of the pigment may lead to fairly dark variants (reds).
The eumelanins are formed from a close relative of DOPAquinone called DOPAchrome. There are two accessory enzymes known that appear to be closely related to tyrosinase, and are therefore called "tyrosinase-related proteins", TRP1 and TRP2. The first is produced by the brown (B) gene in all mammals studied to date (except possibly man). The second has been associated with the slaty mutation in the mouse, but there appears to be no equivalent in the poodle. These three enzymes appear to function in the order tyrosinase-TRP2-TRP1, and all three are necessary to get black eumelanin. Homozygous TRP1 mutants (bb) are brown. A true brown must have no black pigment anywhere including the eyes and nose. BB and Bb are black, if there are no other genes giving contrary instructions.
Do we ever get mixtures? (the E gene)The relative amounts of eumelanin (whether black or brown) and phaeomelanin synthesized are under the control of a protein, called MC1-R, in the melanocyte's cell membrane. The function of this protein is to take a signal, such as melanocyte stimulating hormone (MSH), coming from another tissue or cell, and pass that signal on to regulatory factors inside the cell. The nature of these factors has not yet been fully determined, but the end result is that MSH stimulation leads to a "diversion" of the tyrosinase products from the DOPAquinone-phaeomelanin pathway to the DOPAchrome-eumelanin pathway. The norm in many animals is a mixture of the two pigments, in which case you get hairs with both pigments visible in bands and/or different colored hairs. However, this is rarely seen in the poodle.
The gene responsible for MC1-R is more commonly known as the extension gene (E). This gene was recently cloned and sequenced. A mutation (e), leading to total loss of function in homozygous (ee) dogs is known in Poodles and several other breeds. As the defective MC1-R protein cannot pass on the signal from MSH, the melanocyyte synthesizes only the light phaeomelanin. Genetic testing for the e allele has shown that apricot, cream and white poodles are all homozygous for this allele (see below).
Two apricot Standards. Left: Peaches (7 months); Right: Lance (3 years)
At the other extreme are mutants that produce an MC1-R protein altered in such a way that the melanocytes think they are getting a signal even though they are not. Only eumelanin is synthesized and, in the absence of other modifying factors, we have a black dog (usually designated Ed). This mutation is dominant and, consequently, is referred to as a "dominant black". Dominant black is known in mice and foxes, but it is not clear whether any of the solid black breeds of dog carry this mutation (there are other ways to get solid black). However, there are many breeds that produce both pigments, and they clearly carry a "normal" allele that allows the melanocyte to respond to the signals coming from other cells.
The controversial Agouti (A) geneThe other gene that has been cloned is agouti (A). This gene codes for another signaling protein, called Agouti Protein (AP), which acts to counteract the effects of MSH on the MC1-R receptor. It is normally synthesized only in the hair follicle.
The agouti gene has a large number of alleles, especially in the mouse, and its synthesis is under complex regulation. If mutation results in complete loss of ability to synthesize AP, the lack of inhibitory action may result in the production of only eumelanin by the melanocytes, even though a functional MC1-R receptor is present. This mutation produces a "recessive black" (aa) in mice, foxes, horses and probably many other mammals that have a solid black variant. The traditional claim by geneticists that the domestic dog has a dominant black allele at the A locus would make it unique among mammals, and is now being reevaluated.
The "normal" (wild-type) allele is generally considered to be agouti (a+), though some consider wolf-gray (ag) to be a more appropriate choice for a dog. Until this gene has been sequenced in a wider range of animals, we will not know how similar or different these alleles really are.
Other alleles include sable (as), where the black and yellow are found in the same hairs, and black-and-tan (at), where they are in different regions. Both are rare in the Poodle and are considered a fault in North America and many other countries. However, in Germany, there is a separate registry for black-and-tan (in addition to the one for black-and-white). An example of the former may be found on p. 42 of Rosa Engler's "Pudel".
Dilute (D), gray (G) and silver (V)The dilute (D) gene is supposed to affect the apparent intensity of the pigmentation, but not through an actual reduction in the amount of melanin present. There are two alleles described in the literature, D, which is dominant and gives full color, and d, which leads to a clumping of the pigment granules in a homozygous (dd) animal. This leads to reduced light absorption.
In an otherwise black animal, the d allele is supposed to produce a "Maltese" blue (slate gray) animal, and possibly cafe-au-lait when acting on a brown. Confusion between the effects of this gene and that of the graying and silvering genes (see below) is common. The Maltese blues are said to be born blue. However, these seem to be much less common than the silver-blues, at least among the Standard poodles.
Left: Paavo, a black poodle from Finland. Center: Sadie, an older poodle showing a typical age-related graying. Note the white hairs in the ear. Right: Cassie, a five-year old gray.
Most authorities describe a dominant allele (G) for graying; non-gray would be gg. Some also consider it to be the gene for silver, in which case it would have to be a partial dominant. Willis (1989), however, says that silvers are dilute grays (ddG_; he does not indicate whether ddGG and ddGg would be the same). Searle (1968) says simply that "this dominant gene apparently leads to a progressive graying in coat-color throughout life and seems to be present in poodles."
My own study of standard poodle pedigrees is consistent with the interpretation that gray and silver are separate genes. To avoid confusion, let's call the silver gene V . This gene shows incomplete dominance. In other words, if a poodle that is VV would be black, Vv would be a dark blue-gray and vv would be silver. Both blues and silvers are born black. Silvers "clear" during the first year. This involves the gradual loss of pigmentation from about 90% of the inner coat (hairs become transparent or white, depending on thickness), but a substantial percentage of the outer, guard hairs retain some color.
Left: Paul, a blue at about one year. Center: Indy, an 18 month old silver poodle. Right: Julie, a three-year old silver. (Not all silvers are this light.)
To better understand the differences between these colors, Kelly Cassidy and I obtained hair samples from 50 poodles ranging from black to cream. The samples were examined under a microscope. Colors were scored as black/dark brown, medium brown, light brown, or white (which also includes finer transparent hairs). The average percentages for each hair color are indicated in the table, below.
The V gene also affects brown and apricot, as follows:
Cream and WhiteIf you have managed to stick with me this far, you will likely have noticed that two different genotypes have been mentioned as possibilities for cream: eeVv and cchcchee. Evidence for the first comes from crosses between silvers that carry the apricot gene, and apricots. Such a cross produces blues and creams. Lets take a look at this more closely:
Silver, Eevv x apricot, eeVV => 1/2 blue, EeVv, and 1/2 cream, eeVv.
However, creams are also obtained in crosses between two blacks, often at close to the expected 1/4 for a recessive trait, and with no other colors than black and cream appearing. These cannot be silvered apricots, as at least one of the parents would have to be blue, and both blue and apricot progeny would be expected in addition to black and cream.
Rufus (R)Red poodles are rare, generally appear in apricot lines, and appear to be the result of a separate gene. Willis, citing Robinson, talks about "rufus" genes, that are poorly characterized, but may act to darken an apricot or brown coat. As the poodle pedigrees for reds suggest only one such gene, I propose that it be called F (rufus; R is already used for roan). The recessive allele, f, produces red in an apricot (i.e. eeff), and may also affect brown, but is supposed to have no effect on black.
What does the future hold?The development of coat color is a complex process. Even if there were no connections to health problems such as obesity and deafness, research would likely continue simply because it provides a good model for understanding regulatory processes in development and good examples for illustrating basic genetic principles.
The importance of the E gene has been clarified by recent molecular studies and the function of the A gene is becoming clearer. Their function appears to be one of balancing the different melanins to achieve an overall effect that provides an element of camouflage to the wild canid and other mammals.
The E-gene product, the MC1-R protein, is unusual (but probably not unique) in having both positive (MSH) and negative (AP) regulators. The Agouti protein was unique, but having sequenced the gene, geneticists have already used it as a kind of molecular "fishing hook" to identify similar genes in man. As more genes are found, no matter what the species, all will benefit from these "fishing trips".
Photo creditsCassie and Sadie (gray): Diane Whitehouse; Gus (Parti): Elizabeth Glew; Indy (silver): Jo Kendall; Julie (silver): Cheryl Leibowitz; Lance (apricot): Sharon Ciardullo; Paavo (black): Pirkko Ranta-aho; Paul (blue): Janice Bennett; Peaches (apricot): Darrell Fritz;
Searle, A.G. "Comparative Genetics of Coat Colour in Mammals", Logos, London, 1968.
Vage, D.I. et al. A non-epistatic interaction of agouti and extension in the fox, Vulpes vulpes. Nat. Genet. 15: 311-315 (1997).
Willis, M.B. "Genetics of the Dog", Whitherby, London, 1989.
© John Armstrong, 1997, 1999
Revised June 20, 1999