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B/b, E/e, and Beyond:

A Detailed Examination of Coat Color Genetics

in the Labrador Retriever

Why do yellow Labs have variations of shading? What causes the fox-red color in yellow Labs? Why do some Labs have white spots on their chests? What causes some Labs to have white at the base of the hair shafts on their tails? Why do many Labs develop gray hairs as they get older? The answers to these questions and many others pertaining to coat color can be found in the canine genes that influence production of pigment, also known as melanin. These same genes determine the black, chocolate or yellow coat color found in the Labrador breed, however, to address these former questions, one must explore beyond the genes responsible for simple solid coloration. The following article presents a detailed examination into the factors responsible for inheritance of coat color including an explanation of the modifying genes responsible for unique coat-color attributes.

Table of Contents


Black Puppies Produced from Two Yellow Parents?...Or From Two Chocolate Parents?..."Impossible!" You Say?...Maybe Not!...

    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. However, in some other breeds, homozygous "ay" at the Agouti loci is recognized as being responsible for producing the yellow (tan/sable) color in dogs with the wild-type (E+) Mc1r, such as in the Basenji, Collie, Dachshund, etc.

    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. This observation was first made by Clarence Little in 1957 and later confirmed by Burns and Fraser in 1966. Because these test breedings were controlled studies, the possibility of mismating as an explanation was ruled out and a new hypothesis was postulated: There are two kinds of "yellow" Cocker Spaniels, an AsAsB_ee Cocker Spaniel that is usually buff-colored and an ayayB_E+E+ Cocker Spaniel that is usually sable-colored. When these genotypes are crossed, one possible resulting genotype of the offspring will be AsayB_E+e: a black Cocker Spaniel!

    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. the bitch was exposed only to the intended stud and there was no opportunity for breeding to occur with any other male). 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 "ay" 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 "ay" (refer to: "Canine Color Genetics" by Sue Ann Bowling). 

    It is possible that like the Cocker Spaniel breed, the Labrador has two genotypic "kinds" of yellow dog: one that is homozygous "e" (more common) and one that is homozygous "ay" (less common) with the wild-type Mc1r (E+). 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 ayay 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).  As such, crossing of one of these chocolate-appearing yellow Labs with a true chococlate would produce AsayBbEE+: a black Lab.

The E+ allele: Enter the Potential for a "Gain of Function" Mutation in the "e" allele of the Extension Locus

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. Crossbreeding to breeds that carry the "wild-type" extension allele (E+) as well as the possibility of a spontaneous mutation resulting in a "gain of function" (also denoted E+) of the Mc1 receptor encoded by the recessive "e" allele, may be possible explanations for color oddities occurring in the breed.

The E+ allele (whether wild-type or "gain of function") would encode a normal, functional Mc1 receptor that would be under greater influence of the alleles at the Agouti (A) locus. Unlike the E allele mutant found normally in Labs that does not require activation by MSH to produce eumelanin (and therefore is always "turned-on" producing black or brown pigment), the Mc1 receptor encoded by E+ would be dependent upon MSH for production of eumelanin (black/brown pigment). Homozygous E+ would allow the effects of the Agouti locus that would otherwise not be seen in a Lab carrying the typical mutant "E" allele (in the typical Lab, effects of Agouti are only seen if the dog is homozygous "e"). Therefore, a Lab that is homozygous E+ would also have to carry As to appear solid black (or chocolate dependent on the allele at the B locus). If a recessive allele (ay, at or as) were the most dominant allele at the A locus, then the Lab would appear either red/yellow (ay or as) or black-and-tan (at). Other Agouti alleles such as aw (which is attributed to producing silver in some breeds) may also be observed.

    The range observed in silver coloration suggests that silver occurs through a modifying gene. There have been several possible outcomes observed for the silver Lab:

Black Lab + silver modifier = charcoal gray coat with a "sparkly"-like appearance. Nose: dark gray; Eyes: dark to light gray

Chocolate Lab + silver modifier = "mousy"-brown gray coat. Nose: same as coat; Eyes: yellow to gray-yellow

Yellow Lab + silver modifier = platinum to pale silver (yellow with gray casting). Ears: gray (instead of red-toned); Nose: dark to pale gray; Eyes: dark to pale gray.

An alternative explanation for explaining the modification of both eumelanin and phaeomelanin again returns to the wild-type/gain-of-function "E+" allele that encodes for a normal functioning Mc1 receptor. If this allele either occurred as a spontaneous mutation or was introduced into the breed through interbreeding, this might explain the modification occurring in all three colors, particularly when one considers the following:

When one traces the pedigrees of some silver Labs, one finds a history of other color oddities occurring in some related bloodlines to the silver Labs. Occurrences of "black-casting" in chocolates, muted chocolate coloration ("card-board box" coloring), as well as the occasional occurrence of black puppies being whelped from two chocolate parents suggests that these "chocolates" were probably not chocolate at all but rather E+ yellows. As such, it is conceivable that the Agouti alleles could produce an intense red pigment resulting in deep red (interpreted as chocolate especially in the absence of "saddling" modifiers) or diluted, muted red (card board box color) due to further modification by the alleles of the C locus). In black Labs, an ayayEE+ geneotype could produce a muted black color (because of the presence of both receptor types) especially if the alleles at the C locus were cch, thus resulting in a deep charcoal, silvery coat appearance. This suggests a possible role of E+ for the silver coloration as well as for a multitude of other coat color variants that occasionally occur in the breed.