Dog Genetics 2.0: Colours
Coloration is a physical trait of dogs that is visible, is not associated with disease conditions (with some exceptions), and that has been desirable since the beginning of the development of dog breeds. The genetics of dog coloration is a good practical example of basic genetics and heredity. We will see the function of single genes, the passage of genes to the next generation, and importantly, the interaction of genes with each other to give the combinations of color traits seen between and within the dog breeds.
The basics of color genetics in the dog are relatively simple, but be careful as simplicity plus simplicity plus simplicity….gives us complexity. This brief review of the genetics of coloration in the dog should be considered a starting point, designed to simplify what can become quite complex. More complete coverage of dog color genetics can be found elsewhere (see links at end).
The genes that contribute to the colour and the patterns of colour seen in the dog can be grouped into four levels of function:
- Genes for basic colours, including Agouti (Locus A), Extension (Locus E) Locus K and Locus C. These are the genes that make the pigments that give the basic colour to the dog.
- Genes for modification of the basic colours, including Brown (Locus B) and Dilution (Locus D). These are the genes that modify the colours defined by the basic colour genes.
- Genes for white spotting, including White Spotting (Locus S). These genes don’t make colour, rather they are responsible for the lack of colour (i.e. white).
- Genes for colour patterns. These genes include Agouti (Locus A, which also contributes to basic colours), and Merle (Locus M).
Keep in mind that genes come in two copies, i.e. our genetic system is in duplicate (see Dog Genetics 1.0: The Basics). Also, the effect of one copy of a gene can be dominant (with obvious effects) over the other copy, or recessive (with potentially hidden effects). For a given gene an individual dog can have two copies that are the same (either two dominant or two recessive copies), or one copy that is dominant and one copy that is recessive. Furthermore, the functions of one gene can mask the functions of another gene.
Genes for Basic Colours
Colour is produced by special chemicals called pigment. Pigment is produced in special cells called melanocytes, which are found within the hair, in the skin and in the eyes. For the dog, the colour and pattern of the pigment within the hair is the most apparent.
The two basic pigments that contribute to coloration are black and yellow pigments. It is the interplay between the expression of the genes responsible for these two basic pigments that give the dog its basic coloration. It is important to realize that a single hair can be all black, all yellow, or yellow with a black band at the tip.
Locus K (dominant black)
Locus K is unique to dogs and is not found in other domestic animals including the cat. A dominant version of Locus K (Kb) is responsible for the Dominant Black coloration that is seen in dogs such as the Labrador Retriever. A second variation at the K locus is responsible for the Brindle coloration (Kbr) seen in some breeds such as the Boxer. Finally, a recessive version of K (Ky) allows the Agouti gene to be functional.
Locus E (recessive yellow)
Locus E, also known as Extension, affects black pigment production. When dogs are double recessive at the E locus (e/e) and also have the Dominant Black (KB) at the K locus, then this gives Recessive Yellow. This is in contrast to the Dominant Yellow seen with the Agouti locus, and can be the source of some confusion. Recessive Yellow is by no means dismissive; it is responsible for the majestic golden coloration of the Golden Retriever, and the regal red coloration of the Irish Setter. Locus E also has a dominant version (EM), which gives the ‘Black Mask’ pattern of coloration in the face that is seen in many dog breeds.
Locus A (Agouti: dominant black and recessive yellow)
The Agouti gene, found at Locus A is the master gene for yellow pigment production. In the dog, the Agouti gene is responsible for Dominant Yellow (sable), Recessive Black (a/a), and a couple of color patterns including Agouti (now referring to the pattern of yellow hairs with black tips as seen in German Shepherds), and Black and Tan as seen in many breeds but let’s mention the Gordon Setter because that was our first family dog. It is important to note that for Agouti to be functional, the dog must be double recessive (Ky/Ky) at the K locus.
Locus C (Albino)
Locus C is mentioned for completeness and out of respect for cats. The C Locus is a master gene that controls the formation of both black and yellow pigments. When the gene is mutated and not active, no pigment is formed. A recessive mutation at the C Locus will give a white (albino) dog, but this is not something that is desirable for dog breeders. So Locus C is not of much interest to dog breeders. They generally don’t care that Locus C gives the wonderful Colorpoint traits seen in cats (see Cat Genetics 2.0: Colours).
Genes for Modification of Colors
The color modifying genes do not make the color pigments, but they do influence the shade of the color, usually by reducing (diluting) color intensity. Modifying genes include Brown (Locus B) and Dilution (Locus D).
Locus B (Brown)
The Brown gene (Locus B) in the dog modifies the intensity of black pigment. Brown is a recessive trait that will lighten the normal black pigment (B) to give a brown color (b/b). Normal coloration is dominant over brown. The B locus is not active on the yellow pigment from the Agouti locus.
Locus D (Dilution)
Dilution is another gene that modifies the normal colours, and will wash out or dilute both black pigment and yellow pigment. Dilution is a recessive trait that will modify normal black pigmentation to give a grey color (d/d) called either Blue, Grey or Slate depending on the breed. Dilution can combine with Brown to give additional coat colors in the dog including Isabella, Lilac (as in the cat), Fawn, or Buff, again depending on the breed. In some breeds, Locus D can be associated with alopecia or loss of hair, probably due to another as yet unidentified gene acting in combination with d/d.
Genes for White Spotting
White markings are a prominent feature of dog color genetics. Interestingly, white is not a color per se, but is rather a lack of color. In fact, white results from a problem during development due to a lack of the cells (called melanocytes) that produce pigmentation. The major gene for white markings in dogs is at the S locus, and classically four versions are described:
S – The Dominant version, which gives a dog with no white spots.
si – Irish white spotting gives a dog with white undersides.
sp – Piebald white spotting gives a dog with random white spots.
sw – Extreme white spotting gives an almost entirely white dog, usually with a bit of pigment left on the head.
Recently, the gene involved in the S locus as well as four separate mutations associated with white spotting were identified. However, the genetics of white spotting remains complicated and not fully understood. Indeed, the different white spotting patterns observed may be the result of different combinations of the mutations that were described, and this may vary between breeds. Furthermore, other genes may modify the activity of S. It should be noted that extreme white spotting can be associated with developmental problems such as deafness or heart murmurs, as can be seen with white Boxers. So the final word on the genetics of white spots awaits a future date. For the intrepid souls wishing a more thorough discussion of the genetics of white spotting, I refer you to more comprehensive reviews:
Locus A (Agouti)
The Agouti gene does double duty, acting as a master gene for yellow pigment formation, but also involved in color pattern formation. This pattern can be the banded hair pattern (light shaft, dark tip) seen in German Shepherds and wolves, or in the black and tan pattern of yellow pigment seen in many dog breeds.
Locus M (Merle)
Merle is a striking irregular color pattern seen in a number of breeds of dog. The Merle pattern gives mottled washed out patches of hair color in an otherwise solid coat. What should be known about Merle is that it is a gene that acts during development (a bit like the S locus), that it is a dominant trait (only one copy needs to be mutated to give the Merle coat pattern), and that it can be associated with birth defects including eye and ear problems. What should be done about Merle is never mate two Merle animals together, as double Merle animals (both copies of Merle mutated) have an increased chance of having birth defects.
Once again, this presentation of dog color genetics is designed as an initiation to the field. Many interesting points have been left out, including ticked patterns, progressive greying, the genetics of short, long and curly hair, among other topics. This has been done on purpose, to avoid the problem of “losing sight of the forest because of all the damn trees”. More comprehensive and in depth coverage of the topic of dog color genetics is to be found elsewhere:
© 2018 David W. Silversides