The Dog Beautiful Genetics and Inheritance

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The Genetics of Coat Color

Posted by Theresa Wright, Ph.D.  •  Filed under Genetics, Facebook

The domestication of the dog has brought out the expression of the "zillions" of hidden colors variations and combinations that were selected against in the wild because "different" coloration is otherwise disadvantageous in the wild.  Any noticeable differences in coat color in the wild become targets for predators and are usually eliminated before having the chance to reproduce.

The wild wolf as well as the natural progenitor of the Northern Breed dogs carry the potential to express all of the various color patterns are present in the native genome. Of course, the most common coat color is the wolf is gray-ish which is actually not solid, but of mixed colors, such as gray/black and a variety of color shadings that are the most common.  Solid colors such as black or white appear with low frequency because these colors and patterns make them more vulnerable to predators.  The most interesting aspect of the expression of coat color is that all colors and combinations are a representation, extension or modification of the agouti coat color gene.  The agouti gene produces color banding, which is alternate banding of light and dark on same hair shaft.

Coat color, skin pigment and eye color all require the presence of a substance called melanin.  There are two types of melanin: eumelanin and phaelomelanin.  Eumelanin produces black and brown pigment, depending on the concentration of melanin.  Phaelomelanin produces reddish or yellow color.  The arrangement of these pigment granules differs in shape, size, number and color to produce the variation of shadings as well as the different colors.  Actually, all external coloration is produced by melanin. The complete lack of melanin results in translucent white (for lack of a better descriptive color) hair, very pale translucent skin, and red eyes.  The skin and eyes are often thought of as pink, but that coloration is due to the underlying capillariy blood flow, not pigmentation.  The term albino is used when melanin is totally absent. 

White coat color in the Northern Breeds is a polygenic trait.  This means that color expression is under the control of several different alleles, not just one or two.  To complicate matters, there are genetic modifiers that can lighten or darken natural coat colors. which makes the determination of the base genotype a very difficult task. The influence of the modifiers makes the mode of coat color inheritance very inconsistent in some breeds.  There are at least 6 alleles (and several modifiers that work in random fashion) that combine to form white coat, black pigmentation and brown eyes.  Biscuit markings in the white coat are also possible.

The mode of transmission of coat color combinations can be determined by focusing study on the genotype combinations in the northern breeds, such as the American Eskimo, the Spitz breeds, the Norwegian Elkhound, the Pomeranian, the Skipperke and Samoyed.  Extending this study by looking at the mode of inheritance in breeds that carry white coat color, such as the Pyrenean Mountain dog and the German Shepherd, a distinct and tenable pattern emerges!

 The Allele Series

There are several alleles involved in the coat color series for all breeds of dogs.  An allele is a single genetic component that exists at a specific locus.  A single allele is inherited from each parent.  If the alleles are identical, the gene is homozygous.  If the alleles are different, the gene is heterozygous.  The dominant allele in the pair is expressed and the recessive allele (unless homozygous) is hidden.  

The alleles and their attributes and the associated variants are summarized in the following table. Capitalization of the allele confers dominance, small case represents the recessive.  The order of dominance in each series is in descending order.

For example, AA and Aa will both express the dominant trait.  The recessive trait a is hidden and not expressed.  The pairing of aa is homozygous and the recessive trait is expressed and shown.  This is simple Medelian genetics, however, coat coloration becomes far more complex. 

The Allele series in the table descripbes the variations of dominant and recessive genes that exist in the dog genome.

 

Allele	Allele Series	Expression Type
A	Agouti Series	Wild type coat color in which an individual hair has black pigment (eumelanin) at the tip and the base, and a band of yellow pigment (phaeomelanin) in between.
	A	Dominant Black
	ay	Golden Yellow/Tan
	ag	Wolf-gray
	as	Forms a black/liver saddle
	at	Bi-color black/liver and tan body
B	Black	The B allele only enables the formation of black pigment.  This allele is not expressed as a particular color. Determines type of melanin formed.  (Eumelanin or phaelomelanin.)
	BB Series	The color expressed depends on the presence of A, C, D and S alleles
	bb	This combination is recessive and disables the formation of black pigment, resulting in chocolate or liver.  Red or yellow is produced if a genetic modifier is present. Is associated with  lighter eye color.
C	Albino Series	The C allele only enables the formation of melanin.  Most breeds are CC.
	cch	This is known as the chinchilla allele which lightens tan pigment to a pearl-gray.  The effect on black pigment is rarely noticeable.
	cd	White coat with black nose and dark eyes
	cb	Cornaz coat (pale gray coat) with blue eyes
	cc	Albino - complete lack of pigment.  Complete inability to form melanin.
D	Dilution Series	This allele works on the other allele to cause a different color expression.  The D allele will allow the other alleles to express their natural color.
	dd	Dilutes (lightens) the color of coat, skin, and eye pigment
E	Extension Series	This allele refers to a mask, which relates to the coloration around the face, muzzle and head. Permits black color formation.  (Dog with mask is dressed up for Halloween)
	E	The E allele permits black coat color if the Black allele is present, but does not permit black in the mask.
	em	Causes a dark mask
	ebr	Causes a brindle color mask and coat
	e	This is a restriction allele that does not allow black to be formed even if the Black allele is present.
G	Gray Series	Black at birth.  Changes with age to gray/bluish color
M	Merle Series	Merle coloration of the coat.  (MM is associated with defects such as wall eye, deafness and blindness.
P	Depth Series	Factors that influence of depth of penetration of the pigment on the hair shaft or skin cell
S	Spotting Series	Responsible for white spots
	S	Self color - totally pigmented
	si	Irish spotting - white patches in predicable, repeatable pattern on body (Collies, Basenjis, and Boxers)
	sP	Piebald spotting pattern. Larger percentage of white color on body in progressive pattern. (American Cockers and Beagles)
	sw	Extreme white piebald - Solid White
T	Ticking	Produces non-white spots (usually liver or black, as in the Dalmatian.)
Modifier	Rufus polygenes	Can lighten or darken the shades of liver, chocolate, etc., as well as modify eye color.

 

Centers of Pigmentation

There are several "Centers of Pigmentation" on the dog that inherit coat color independently.  The alleles are located at specific loci, which govern the size and arrangement of the pigment granules in the hair shaft and results in coloration. 

The genotype for breeds that do not have any white color is SS, which provides full color expression. The s (recessive) allele series is responsible for the appearance of white coat color on the dog. Genetic studies of white show that white first appears on the tip of the tail, along with the toes, chest, belly and/or muzzle. The characteristic patterns for white progresses in an orderly sequence until the dog is completely white.  White progresses up the forelegs, the hind feet and around the neck. The last areas to become white are the ears, head and around the eyes. 

This explains the appearance of biscuit or buff around the head, ears, eyes, and down the back.  A completely white coat is the action of the extreme white piebald (s^ws^w) gene. White coat color is recessive to the solid coat colors and the allele must be homozygous in order for white to be expressed.  The s^ws^w gene hides the color that would normally be expressed by the A series alleles.

Genetic Modifiers

Two genetic modifiers affect white coat color.  The 'Plus' modifier produces less white.  The 'Minus' modifier produces more white. This can lead to an overlap between genotypes, and also causes a great deal of obscurity when attempting to determine the true genotype.  Even though S is dominant, dogs with the Ss genotypes can have white markings. 

Markings that lighten or disappear before adulthood are acted upon by minus modifiers that act on the s allele. These modifiers can be influential enough to produce white areas in genetically solid black coats. If plus modifiers are present, color (usually yellow, tan, or biscuit) occurs around the ears, face, and eyes.

The series G, M and T do not have any significance to white coat color.  These alleles emerged as the specialization of crossbreeding for specific types progressed further.  The P gene is still under study.

 Genotype Combinations

Genotype	Color
ccee	White coat & blue eyes (not a true albino, but less melanin formation)
Dd - with BB or Bb	Black
Dd - with bb	Red
dd - with bb	Fawn/tan (a dilution of chocolate/brown.)
dd - with Bb or BB	Blue (a dilution of black.)
ee - with B -D	The combination calls for black color, but black is suppressed except for the nose and eyes
ee - with cchcch	White or yellow/white
ee - with ay	Yellow
ee - with as or at	Disallows black coat color and produces tan

 

The Genetic Color series mapping has been established for the breeds listed in the table below:

 

Breed/Color	Genotype
Keeshond	Wolf-grey: agagBBCCDDEEggmmSStt     (or agay) Orange sable: agay (a cross between tan and wolfgrey.)    * Light colored areas are DD (not cch)
Samoyed	White: ayayBBcchcchEEggmmswswtt           (Tan color is lightened by sw & cch to white.)  White: AABBcchccheeggmmswswtt  * Rare occurrence of Blue eyes, possibly due to cc or cbcb.  dd can cause grey eyes.
Skipperke	Blacks: AABBCCDDEE  or AayBbCcDdEe Fawns: ayayBBCCDDEE or AAbbCCddee Chocolates: AAbbCCDDEE
Pomeranian	Homozygous for E, g, m, t, and possible S. Solids:  AABB(or Bb)CCDD(orDd)EEggmmSStt Black & Tan: atatBB(or Bb)CCEEggmmSStt Pale Sable: ayayCcch(or cd)EE(or Em)ggmmSStt Wolf Sable: ayayBB + any above combination Orange: ayaybb + any above combination. *The occurrence of Blue eyes has been noted in Poms.
Norwegian Elkhound	Black with grey: agagBBcchcchDDEmEmggmmSStt Liver:  agagbbcchcchDDEmEmggmmSStt
German Shepherd	Black/tan: asatBBCCDDEEggmmSStt  (Possibles are: ag, b, cch, e.) Brown with light colored eyes attributed to bb Blue coat color attributed to dd White/yellow coat color attributed to cchcchee Black - recessive allele (not produced by dominant A allele.)
Pyrenean Mountain Dog	White: ayayBBcchcchDDEEggmmswswtt. (Aslo possible AAee.)

 

Discussion

The most common coloration is the wolf grey.  Hidden within this genotype is the potential for all colors. In the wild, solid colors appear occasionally.  However, when the gene pool is restricted, solid black, chocolate and white appear with greater frequency.  The smaller the gene pool, the higher the chances are of recessive genes to be expressed. 

The potential to produce a solid black or chocolate has the highest frequency.  The ultimate expression of the recessive genotype is the orange/buff coat color coupled with longer than average coat length such as the Pomeranian pictured on the left.

Color diversity is responsible for the appearance of unexpected colors in an established breeding program.  It's not unusual for a solid white or solid black to appear after 5-10 known generations of purely wolf gray in the Keeshond. 

Eye color is not strictly related to coat color although associations to lighter coat colors are apparent. Dark brown eye color is dominant, however dominance is incomplete and various shades can exist from light yellow to almost black. If a dilution gene is present, eye color is lightened to a blue or a smoky gray color. The bb genotype lightens a dark brown eye to hazel, or lightens a hazel eye to very pale yellow.  The cc genotype will always result in blue eyes. 

 

References:

Genetics of the Dog, by Malcolm B. Willis, Ph.D.,Howell Bookhouse, 1989.

The New Art of Breeding Better Dogs, Kyle Onstott, Howell Bookhouse, 1962.

The Joy of Breeding Your Own Show Dog, Ann Seranne, Howell Bookhouse, 1988.