Beyond the Colored Coat

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To understand these mechanisms, one must go back to the DNA sequence of the gene.

The DNA of each gene locus is responsible for producing a single protein. As mentioned earlier, DNA is composed of molecules called nucleotides. When these nucleotides are arranged in specific order they encode for amino acids, which are molecules that make up proteins. The sequence of amino acids, in turn, will determine protein structure and function. Therefore, for normal cellular function, the correct sequence of DNA all the way through to the sequence of amino acids must be conserved.

When mutations occur in the DNA nucleotide sequence, the resulting amino acid sequence may be altered. As a result, the protein for that gene locus will either not be produced, or will be different from the normal protein. In the latter instance, the alternate protein may function adequately but produce some physiological changes not seen in the presence of the normal protein.

Therefore, different alleles occur as a result of mutations in the DNA for a particular gene locus. In many cases, recessive alleles are those that produce no protein. In other cases or in cases where there are more than two possible alleles for a gene locus discussed below usually there is a rank order of dominance.

In such an instance, the DNA mutation results in a structural change in the protein that influences its reactivity. Put more simply, those alleles that produce proteins with the greater capacity to function will be more dominant to alleles that produce weakly-functional or non-functional proteins. Therefore, in the case of a heterozygous Bb black Lab, the black producing allele produces a protein that functions more efficiently than the protein encoded by the chocolate producing allele. Despite the fact that the B locus on a chromosome may be occupied by either the "B" or the "b" allele, the same molecule responsible for the black coloration in the Lab is also responsible for the chocolate coloration.

Melanin, or pigment molecules, are produced by and packaged into small organelles called melanosomes. Within the melanosomes are enzymes called tyrosinase , which are proteins needed to make melanin from molecules called tyrosine, and structural matrixes upon which the melanin is organized after it is made. The actual color of the melanosomes is determined by the amount of melanin it contains. Melanosomes are produced by specialized cells called melanocytes.

B/b, E/e, and Beyond

Melanocytes are distributed throughout various areas of the body including the eyes, the hair, and the skin where they transfer the melanosomes into the cells that compose these structures. Color of each structure will be determined by both the color of the melanosomes it contains as well as the distribution of the melanosomes within its cells.

In hair comprising the coat of dogs, two types of melanin have been found.

Phaeomelanin is formed by a modification of the pathway that leads to production of eumelanin from tyrosine. It is important to know that phaeomelanin has only been found in cells composing hair and not in cells composing the skin. In Labs, eumelanin and phaeomelanin production are controlled by the E locus. The protein product of other gene loci will determine the level of pigment expression. For example, in the instance of black versus chocolate coat color, color appearance is actually determined by the B locus.

The B locus controls pigment not by controlling changes in the actual eumelanin molecule, but rather by controlling number, size, and pattern of the distribution of melanosomes in the hair shafts. Therefore, Labs that are homozygous for black at the B locus have large eumelanin-producing melanosomes that are evenly and densely packed throughout the hair shaft.

Labs that are homozygous for chocolate have melanomsomes that are less tightly packed into the hair shaft. This locus encodes the melanocyte stimulating hormone receptor a. Labs that are homozygous for the dominant E allele have a constitutively active, mutant form of Mc1r; that is, the receptor is always "turned-on", even in the absence of melanocyte stimulating hormone MSH. As such, eumelanin is constantly produced and the dog appears black or chocolate.

Labs that are homozygous for the recessive "e" allele also have a mutant form of Mc1r. This mutant, however, is a "loss of function" receptor that cannot produce eumelanin, even in the presence of MSH. Therefore, Labs that are homozygous for the "e" allele can only produce phaeomelanin and, therefore, will appear yellow. In some other breeds, the Agouti locus is responsible for determining yellow color.

Some of the recessive agouti alleles produce molecules that inhibit the activation of Mc1r by interfering with binding of MSH to the receptor. As such, these breeds often display a combination of black as well as tan yellow coloring resulting from production of both eumelanin and phaeomelanin. The Agouti genes are often ignored in the Labrador with most writers stating that all Labs are A s at the agouti locus. This information is based solely on the observation that agouti will cause both black and tan banding of the hair shaft and since the black banding is not present in Labs, Labs must be A s the allele that encodes the agouti suppressor.

This argument, however, fails to take into consideration the presence of the mutant "loss of function" receptor in yellow Labs compared to these other breeds. The Mc1r in yellow Labs is unable to produce eumelanin pigment under any circumstances.

Instead, the effects of the recessive Agouti alleles in yellow Labs cause banding of phaeomelanin pigment in the hair shafts and as a result, provides the shading effects observed in yellow Labs. It is for this reason that one only observes the effects of the Agouti alleles in the yellow Labrador. If the yellow Lab carries the dominant Agouti supressor gene, A s , phaeomelanin production will be inhibited, and since eumelanin cannot be made, the dog will appear very pale yellow nearly white.

In contrast, a y or a s agouti will inhibit MSH from binding to the receptor and phaeomelanin production will increase. Some Agouti alleles, such as a s , also produce pigmentation patterns that result in more phaeomelanin production on the the dog's back and less phaeomelanin production on the dog's belly.

Interestingly, the a s allele was not originally proposed by C. Little but was later introduced to explain an allele that would encode for the phaeomelanin saddling effect in some breeds that only have tan pigmentation variation on the back and stomach, as opposed to the black and tan saddling effect caused by the a t allele in some other breeds. At that point in time that the a s was proposed, the prospect of the "loss of function" receptor "e" had not been explored.

However, the a w allele the "white-bellied" allele was identified as influencing the expression of Agouti at different concentrations in different locations of the body. Primarily, dogs carrying a w express more agouti protein for expression of phaeomelanin on the dorsal surface the back and less agouti is produced on the ventral surface the stomach. Further modification of phaeomelanin intensity concentration will be determined by the products of the C locus which control levels of tyrosinase, an enzyme required in the process of pigment synthesis which preferentially acts upon phaeomelanin.

The dominant C allele encodes higher levels of tyrosinase resulting in full intensity of red pigment. Other less dominant alleles encode for less tyrosinase and have the effect of diluting the red pigment to yellow. Getting Down to Business: The actual color appearance of the dog is said to be its phenotype. Simplistically, however, there are at least nine combinations of alleles just to determine which of the three colors the Lab will appear. This combination of genes is referred to as the genotype.

Beyond Fifty Shades: The Genetics of Horse Colors

In the canine, geneticists studying coat color have proposed that there are eleven gene loci controlling coloration. Additionally, each of these loci have multiple alleles. This standard form indicates only the gene loci responsible for producing the black or chocolate coloration the B locus and the yellow coloration the E locus.

For the most part, this system is adequate to predict the expression of simple color traits in offspring produced by two Labs. However, when one wishes to understand more about variations in color intensity or shading within yellows, or the reason for other coat phenotypes that may occur less commonly in Labs, analysis of the other gene loci are required.

These other gene loci are the sites for producing modifiers of coat color; that is, their protein products may alter the size and synthesis of melanin modifiers or influence the distribution of melanocytes distributors to bring about color patterns. The following table indicates the eleven gene loci and their respective alleles currently accepted as influencing coat color in canines and whether or not they influence phenotypes of the Lab.

The Yellow Labrador Retriever: The E locus and its "e" allele encodes the mutant receptor that can only produce phaeomelanin red pigment as observed in the Irish Setter. The product of the A locus and the C locus will determine the location and extent to which this red pigment is diluted to yellow. Therefore, although the "e" allele provides the basis by which the yellow color is apparent, the actual yellow appearance is dependent on the alleles present at both the A locus and the C locus.

Breeders of "true fox reds" will quickly point out that some yellow Labs professed to be "fox-red" are really more dark tan than red and are therefore, not "true fox-reds". The difference in concentration of red color determined by the "a y " or "a s " allele of the A locus is dependent upon the alleles at the C locus. The "C" allele allows for full expression and intensity of red tones, while the "c ch " allele will dilute the red to a clear tan color. The "a s " allele produces the "saddling effects" seen in many yellows in which there appears darker yellow pigmentation on the back, ears, legs, etc.

The "a s " allele also increases intensity of phaeomelanin, but restricts its production to the former mentioned areas on the Lab. The observation that there appears to be no solid fox-red or solid "pseudo" fox-red Labs may be explained by Little's hypothesis that the combination of an "a y " in a homozygous "e" yellow dog is lethal.

If Little's hypothesis is correct, then this would mean that all fox-red or "pseudo" fox-red Labs must be: Medium yellow is probably the most common yellow coloration observed in Labs. The ranges in the shades of the yellow coloration, however, can be quite extensive. Medium yellows, listed from darkest to lightest, are produced by the following genotype combinations:. However, the alleles of the A locus are incompletely dominant, so some phaeomelanin will be produced because of the a s allele.

The phaeomelanin intensity will be further controlled by the "C" locus, hence the intensity of the yellow may be stronger in these heterozygotes carrying the "C" allele and lighter in those carrying the "c ch ". As a result, the coat will appear a cream color and will appear almost white if the "c ch " allele is the most dominant allele at the C locus.

Additionally, Labs homozygous "A s " will show an even distribution of yellow color devoid of shading. In fact, the "white" color may be represented by another allele that may be found at the C locus. The "c d " allele is responsible for producing white hair in other breeds of dogs, like the West Highland White Terrier, while allowing full expression of dark nose and eye pigment.

Though this white color may be distinct from the yellow coloration, it should still be grouped with the other yellow variations since its expression is also controlled by both the E and C loci. Or From Two Chocolate Parents? Geneticists recognize that there are two gene loci that are capable of controlling production of the yellow color in the dog. In Labradors, homozygous "e" at the Extension loci is considered the predominant genotype for producing yellow. The yellow buff color of the Cocker Spaniel was once believed only to be determined by homozygous "e" just as in the Labrador.

Interestingly, however, upon occasion when two yellow Cocker Spaniels are bred, a black puppy will be produced.

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This observation was first made by Clarence Little in and later confirmed by Burns and Fraser in Because these test breedings were controlled studies, the possibility of mismating as an explanation was ruled out and a new hypothesis was postulated: This scenario may not be limited to the Cocker Spaniel breed. Occasionally, black puppies are produced from yellow Lab X yellow Lab crosses.

Some Lab breeders immediately cry "mismating", however, mismating is clearly not the only explanation since many times mismating is ruled-out by virtue of circumstance ie. In addition to what has been observed for the Cocker Spaniel, there may be additional indications supporting this theory. One author has suggested that the way to distinguish between a homozygous "e" yellow and a homozygous "a y " yellow is to examine the whiskers: If the whiskers are cream or straw colored the dog is homozygous "e", if the whiskers are black then the dog is homozygous "a y " refer to: It is possible that like the Cocker Spaniel breed, the Labrador has two genotypic "kinds" of yellow dog: As such, crossing these two different genotypic types of yellow Lab could produce an occasional black puppy from two yellow parents.

This may also explain why occasionally a black puppy is whelped in litters from a chocolate to chocolate cross. It is also conceivable that some of these a y a y Labs, especially if they are homozygous "C" at the C locus may appear to be chocolates rather than yellows albeit with a more red-tone than a brown tone. Why is it that the yellow Labs' ears are always darker than their bodies, even when they have no shading on their bodies? And what causes the noses of yellow Labs to fade during winter months? The answers to both these questions can be found by examining the tyrosinase enzymes responsible for producing melanin from tyrosine.

Some forms of these enzymes are temperature-unstable mutants that only produce melanins under ideal temperature conditions. Some tyrosinase enzymes work more efficiently in colder temperatures. Extremities, like the ears, are usually a cooler temperature than other parts of the body and as a result, the tyrosinase is able to produce more pigment in this region.

Conversely, the tyrosinase enzyme responsible for producing the dark nose pigment in yellows is unstable at low temperatures. Under conditions of low temperature, the tyrosinase enzyme stops driving the chemical reaction, and tyrosine conversion to eumelanin in the skin will occur at a much slower rate. As a result, pigment will fade.

Though certain drugs may also produce pigment fading, this latter cause for reduction in pigment occurs because these drugs will bind to dopa an early precursor to melanin in the reaction from tyrosine to melanin and inhibit the further chemical reactions that result in melanin. This condition will also cause fading of pigment in the Lab.

Color oddities have occurred occasionally throughout the breed history of the Labrador Retriever. Such variations on the typical black, chocolate or yellow coloring have included but are not limited to black-and-tan points, brindling, and silver-casting. It is important to recall that during the early and perhaps mid-history of the breed, interbreeding with other breeds occurred.

Other Agouti alleles such as a w which is attributed to producing silver in some breeds may also be observed. Early breeding records indicate that a Labrador puppy with tan points on the ears, muzzle, and above the eyes as found in the Doberman and Rottweiler would occasionally be whelped to pure-bred Labrador parents. Breeders attributed this to previous interbreeding of Labradors with Gordon Setters during the early history of the breed.

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