Color Genetics

WARNING! It is important to understand the genetics behind breeding these "exotic" color yorkies, and more importantly, that your breeder understands them. These are mutations of the standard color yorkie, simply developed over time by breeding so that these normal recessive, inherent genes display physically rather than remaining hidden in the DNA. However, due to coat texture that is associated with the color manifested, breeding must be done carefully to maintain the desired coat. A quality yorkie is known for its silky coat, and though there are variations within the breed on the texture, it should be as full as possible. The prized silk show coat is the highest quality, with coats running from "silk" to varying degrees of "soft". Prices with breeders will vary depending on the color and texture of the coat. It is understood that effects on health and temperament have already been researched and we are simply discussing genetics in regards to colorations of coat. 

Consider the below information as a quick run-thru of the genetics we are dealing with. It is by no means complete, but covers the colors discussed and their underlying genes:
Every dog has two genes from each locus, with multiple loci for each dog. They can have two of the same gene (homozygous) or two different genes (heterozygous). A heterozygous dog will usually express whichever of its genes is the most dominant. Occasionally incomplete dominance occurs (ie A locus tan markings, white markings), and this means that the dog expresses the most dominant gene but is also partly affected by the less dominant one. So a genotype of A yat (one copy of sable, one copy of tan points) would result in a dog with more black hairs than a homozygous sable (AyAy). 

Also, remember the two types of pigment - eumelanin (black, liver, blue, isabella), which affects the coat, nose and eyes, and phaeomelanin (red), which affects only the coat. Understand that "red" is referring to the correct genetic name, not what you see physically (variations of "red" are red, tan, cream, fawn, blond, etc). Each locus below states which it affects. All locii have not been mapped, so it is still a learning science in canines.

A LOCUS ("agouti series", affects distribution of eumelanin and phaeomelanin)
- Ay: Sable (red with or without black tipping). Formerly labelled ay when dominant black was believed to be above it on the A locus, but now black has its own locus (K) and sable is the most dominant on its locus. 
- aw: Agouti (banded hairs). Contains 4 known alleles with general dominance established from yellow to black. Like sable above, but the hairs all over are banded with black. Most likely the gene responsible for wolf grey. Very little is known about it and its dominance. 
- as: Saddle-marked (red with black markings on the back, neck and tail). Controversial, but many people believe it exists. It does appear to be dominant over tan points and not over sable. Theorized that it has incomplete dominance over tan points, producing creeping tan when a dog has one tan marking gene and one saddle gene. 
- at: Tan points (black body with red on muzzle, chest, eyebrows, legs and vent). Only dominant over recessive black (very rare), so not seen usually. 
- a: Recessive black (solid black with no red in the coat at all). Rare.

B LOCUS ("liver series", affects colour of eumelanin)
- B: Normal pigment. A Bb or BB dog produces normal black eumelanin. 
- b: Liver pigment. A bb dog produces liver eumelanin instead of black.

C LOCUS ("albino series", affects intensity of phaeomelanin, and sometimes eumelanin)
- C: Normal pigment. A dog with one copy of the C gene produces normal phaeomelanin (rich red/tan). 
- cch: Chinchilla. Chinchilla causes lightening of phaeomelanin (red) to light gold or cream, but has no affect on eumelanin (black etc). There may be more than one form of chinchilla, accounting for the extreme variations in shade of red pigment. When present in double dose removes most or all of the phaeomelanin pigment with only a slight effect on black pigment. Black and silver replacing black and tan, or a wolf-like color without the extra banding (see aw, above) may also be due to a cchcch genotype. Dogs with very light tan probably are cchcch or something similar. Brown dogs show lightening even of eumelanin pigment and are thought to be due to a bbcchcch genetic makeup. The possibility of more than one form of chinchilla in the dog needs to be considered - rabbits are thought to have three.
- ce: Extreme chinchilla (or extreme dilution). A stronger form of chinchilla, which causes phaeomelanin (red) to become white, leaving eumelanin unaffected (so eyes and noses remain fully pigmented). 
- cp: Platinum (or ivory). True albino is c, the bottom recessive on the C locus. However, this gene is not thought to occur in dogs. Platinum does though, notably in some breeds more than others. It dilutes phaeomelanin and eumelanin to almost white (often a creamy shade).

There may be a further gene on the C locus which restricts phaeomelanin to the back, tail, neck and head, leaving white markings placed roughly in the same position as A locus tan markings, as in the Shiba Inu. It is possible that this gene is affected by temperature (so restricted to the warmer parts of the dog). The himalayan gene (Ch) is a temperature-affected C-locus gene which occurs on cats, but this results in the opposite effect - colour is restricted to the coldest areas (legs, tail, face).

D LOCUS ("dilution series", affects intensity of eumelanin)
- D: Normal pigment. 
- d: Diluted pigment. When homozygous (dd), turns black to blue and liver to isabella.

E LOCUS ("extension series", affects distribution of eumelanin)
- Em: Masked (black on the muzzle and the ears, and sometimes spreading to black tipping on the chest and/or back). 
- E: Normal extension (no restriction of pigment). Usually written with a capital letter, but is in fact recessive to Em. 
- e: Recessive red (solid red all over, except for white markings). An ee dog is unable to produce any eumelanin (black) in its coat. Any black present will be turned to red. The eyes and nose are, however, unaffected (a recessive red dog may still have a black nose).

(The distribution of colored hairs determined by the A locus is complicated by interaction with alleles at the E locus).

G LOCUS ("greying series", affects how eumelanin keeps its itensity over time)
- G: Progressive greying. A dog with one or two G genes will be born dark-coloured and its hair will lighten over time (eumelanin only). 
- g: Normal (no lightening of pigment).

H LOCUS ("harlequin series", modifies merle)
- H: Harlequin. Areas between patches on a merle dog are turned to white, leaving solid pigmented patches on a white base. Only works with merle gene and does not affect non-merle dogs. 
- h: Non-harlequin (normal expression of merle).

K LOCUS ("black series", affects eumelanin)
- K: Black (solid black all over). Overrides A (agouti) series. Any genes on the A locus will not be expressed. 
- kbr: Brindle (black stripes on a red base). kbr is dominant over k, so a dog only needs one kbr in order to be brindle (but will be overriden by one K gene). Brindle dogs will express whichever genes are on their A locus, but the red parts of the coat (phaeomelanin) will be brindled (black parts will not be affected). 
- k: Non-solid black. A kk dog will express whichever genes are on its A locus.

M LOCUS ("merle series", affects intensity of eumelanin)
- M: Merle (black patches on a grey base). Dilutes random sections of the coat to a lighter colour, leaving patches of full pigment. Phaeomelanin is not affected - only areas of eumelanin can be merled. 
- m: Non-merle (normal expression of eumelanin).

S LOCUS ("spotting series", affects distribution of all pigment)
- S: No white (all of coat is pigmented - no white spotting). 
- si: Irish spotting (white on muzzle, neck, chest, feet and tail tip). Varies greatly, and there may be an allele which causes less white spotting than this one, but it hasn't been identified yet. There is also thought to be a separate gene that causes "true" irish spotting (much less variable than the general type) in breeds such as the Boston Terrier. This gene has been found in a number of breeds but its relationship to the spotting series and the si gene is unknown. 
- sp: Piebald (over 50% white, with large pigmented patches on a white base). These dogs are predominantly white with patches of color. They can be either sp/sp or sp/sw. These dogs are often incorrectly called "parti" (of which I am also guilty of).
- sw: Extreme white (a piebald with a high percentage of white. Colour generally confined to base of tail and head). These dogs are almost all white, with only small patches of color. Because this is the most recessive allele, it means the dog has 2 copies of sw/sw. 
(All of the white spotting genes are thought to have incomplete dominance over one another. This means that a dog with, for example, one gene for no white and one gene for piebald will have an intermediate amount of white - somewhere around the level of irish spotting).

T LOCUS ("ticking series", affects distribution of all pigment)
- T: Ticking (white areas are ticked with small flecks or spots of colour). Ticking is whichever colour would have been on that area if the dog did not have white. Suspected to be another case of incomplete dominance - a TT dog has heavier ticking than a Tt dog. 
- t: Clear white (no ticking on white areas).

Now that was all easy, wasn't it?