Sighthound Domino – Grizzle (e^G)

The e^G allele at the E-Locus modifies any regular coat color pattern by reducing eumelanin and increasing phaeomelanin production. This causes different new phenotypes mainly found in sighthound breeds where it is mostly known as grizzle or domino.

What is Domino or Grizzle in Sighthounds?

Domino changes any background pattern by decreasing eumelanin expression and increasing phaeomelanin expression.

The resulting phenotypes go by various breed-specific names like grizzle, domino, chinchilla, etc… In Salukis it is known as grizzle and in Afghan Hounds it was named after a famous show dog of the 1950s named Tanjores Domino.

Grizzle domino affects any pattern caused by a dog’s K-Locus and A-Locus. The variant e^G is considered one of the rarest trait-associated variants in dogs^[1,2].

It basically changes almost all known patterns, but the resulting phenotype varies greatly depending on what pattern a dog would normally have had.

Similar to northern domino grizzle also reduces dark shading and dark hair banding in any given pattern. It turns all areas pigmented by eumelanin smaller and causes pale undercoat, white whiskers and pale hair roots. Tan markings typically fade to a lighter color and get bigger than expected.

Dominant black (e^G/- K^B/-) dogs turn greyish and express some of their normally masked agouti gene pattern. Brindle striping (e^G/- k^br/-) fades to some degree and turns up in different areas than expected. And any other pattern like shaded sable, agouti, saddle pattern or tan point is expressed with a reduced ratio of eumelanin to phaeomelanin.

Grizzle patterns can sometimes be hard to pinpoint since pattern expression and effects vary greatly between dogs. Additionally, some patterns can easily be confused with regular shaded sable or creeping tan.

The e^G Allele

The E-Locus is part of the mechanism that tells pigment cells what pigment type they should produce. The K-Locus and A-Locus depend on a working E-Locus to express in a regular manner.

Grizzle is caused by the e^Grizzle allele which is a reduced-function variant of the Extension gene. Grizzle domino e^G does not inhibit all eumelanin synthesis like recessive red (ee), but it greatly reduces the amount of eumelanin expressed in any pattern.

The e^G allele has a (incomplete) recessive relationship to melanistic mask (E^m) and wild type (E) and acts (incomplete) dominant over recessive yellow (e).

The authors of the original study describing this new allele put “E^G“ over E in the dominance theory of the E-Locus series (“E^m>E^G>E>e”). They also came to the false conclusion that grizzle might only work on tan points ^[1]. Both of these statements have been proven untrue many times by now but unfortunately are still repeated all over the internet.

Since e^G is recessive to mask and wild type some dogs can carry grizzle without expressing it (E^m/e^G, E/e^G). However, the wild type allele is known to probably only have an incomplete dominant effect over e^G.

Heterozygous dogs expressing the wild type (E/e^G) may show indicators like larger than normal tan points, extreme fading from puppy to adult coat or less than normal eumelanin that a dog indeed carries a domino allele.

But an obvious grizzle domino pattern is only expressed in dogs that are either e^G/e^G or e^G/e. Some breeders think that maybe e^G/e might cause an even paler coat than e^G/e^G implying an incomplete dominant relationship between e^G and e.

Combinations like e^G/e^A or e^G/e^H are extremely rare, so there is hardly any information on the interaction between different domino alleles^[3].

Sighthound Domino Phenotypes

Since e^G affects many sighthound breeds we have a good basic understanding of the effect of grizzle on typical sighthound colors.

Many colors (e.g. liver) or color patterns (e.g. merle, recessive black) are less common or simply don’t exist in sighthounds. So some combinations like grizzle merle or liver grizzle are very rare.

Sighthound Domino and Dominant Black (e^G/- K^B/-)

On a non-domino dog, a dominant black K^B variant would block the A-Locus expression and cause a solid coat color in all pigmented areas. But e^G somehow causes expression of the A-Locus despite a dominant black variant.

On a dominant black dog who is genotypically tan point (e^G/- K^B/- a^t/-) this often causes a shaded grizzle phenotype with a dark overlay on the top and a lighter underside.

And a combination of domino, dominant black and sable (e^G/- K^B/- A^y/-) causes an overall greyish coat sometimes called chinchilla, bronze, blue domino, blue grizzle or simply grey.

Sighthound Domino and Brindle (e^G/- k^br/-)

Brindle adds dark stripes of eumelanin in all areas pigmented by phaeomelanin.

So on a sable coated dog (e^G/- k^br/- A^y/-) the whole coat should be striped while on a tan point pattern stripes would only be visible in the tan markings (aka brindle point).

Since e^G decreases eumelanin production the extent of striping in most brindle domino patterns will be reduced. The remaining stripes will sometimes fade to some pale smudges.

On brindle point patterns (e^G/- k^br/- a^t/-) grizzle domino causes the otherwise black coat areas to express some phaeomelanin. This makes brindle striping visible on a dog’s back and flanks.

Simultaneously e^G often deletes striping from the face and the lower legs where brindle would normally be expressed on a tan point brindle!

Sighthound Domino and Sable (e^G/- k^y/k^y A^y/-)

Grizzle removes eumelanin from any coat pattern and can remove some or all dark shading from a sable coat.

In combination with lighter phaeomelanin and pale undercoat, the e^G effects can add up to create a very light and purely cream or yellow coat.

Affected golden domino or cream domino dogs are sometimes called newsprint cream and can hardly be distinguished from recessive red dogs.

Sighthound Domino and Agouti (e^G/- k^y/k^y a^w/-)

Not many sighthounds with e^G in their breed also come in agouti. While northern domino is often combined with a^w in sighthounds this is a rather rare combination.

Agouti grizzle is sometimes seen in Chart Polski or Taigan.

It causes a widow’s peak and a grizzled phenotype with reduced dark hair banding looking a lot like a regular shaded sable. Some of these dogs have light facial markings with some black coat under the eyes. Dogs can also sometimes have a darker nose bar between forehead and nose leather.

Sighthound Domino and Saddle Pattern (e^G/- k^y/k^y a^sa/-)

In dogs with a saddle pattern, the dark saddle can get very small or even be reduced to only some darker hairs on the back of the dog.

Sighthound Domino and Tan Points (e^G/- k^y/k^y a^t/-)

This is one of the most common grizzle domino phenotypes. Typically it is expressed as a pale face with a widow’s peak or dark nose bar and pale coat on the dog’s underside and darker banded coat on the dog’s back.

A tan point pattern with grizzle (e^G/- k^y/k^y a^t/-) will cause lighter and bigger tan points and create large pale facial markings characteristic for the grizzle phenotype.

This pattern can resemble lightly shaded sable or stay quite dark with only some lighter undercoat and some phaeomelanin expression. In some dogs, this phenotype is even called silver sable, golden sable, etc.

In most dogs pointed grizzle creates a very yellow-greyish and shaded look with a dark overlay on paler than average phaeomelanin.

There are many different names for different phenotypes of this type. Depending on the background color different color combinations are called ivory grizzle, cream grizzle, silver grizzle, yellow grizzle or cream domino, golden domino, cream domino, silver domino, etc.

But in some dogs phaeomelanin will not fade to cream or yellow. Grizzle with normal phaeomelanin expression is sometimes called red domino, red grizzle or deer grizzle.

Breeds with e^G

Grizzle domino is common in some closely related sighthound breeds. It was also confirmed in some other dog breeds that mostly originate from Asia or the Middle East^[2]:

• Afghan Hound
• Borzoi
• Chart Polski
• Saluki
• Taigan
• Tazy
• Hortaya Borzaya
• Silken Windhound
• Silken Windsprite
• Xigou
• Caravan Hound
• Anatolian Shepherd Dog
• Tibetan Mastiff
• Canaan Dog
• Central Asian Shepherd Dog
• Kritikos Lagonikos

In some other breeds, the e^G variant occurs at a very low frequency and cases of dogs actually expressing a grizzle domino pattern should be very rare^[2]:

• Akita
• American Foxhound
• Black Russian Terrier
• Caucasian Shepherd Dog
• Dachshund
• Greyhound
• Large Munsterlander
• Maltese
• Manchester Terrier
• Mi-ki
• Norwegian Buhund
• Norwegian Lundehund
• Pembroke Welsh Corgi
• Pomeranian
• Poodle
• Puli
• Tibetan Mastiff
• Treeing Walker Coonhound
• Yorkshire Terrier

Learn More

Links

[1] Dayna L. Dreger, Sheila M. Schmutz. A New Mutation in MC1R Explains a Coat Color Phenotype in 2 “Old” Breeds: Saluki and Afghan Hound. Journal of Heredity, Volume 101, Issue 5, September-October 2010, Pages 644–649. https://doi.org/10.1093/jhered/esq061

[2] Dreger DL, Hooser BN, Hughes AM, Ganesan B, Donner J, Anderson H, et al. (2019). True Colors: Commercially-acquired morphological genotypes reveal hidden allele variation among dog breeds, informing both trait ancestry and breed potential. PLoS ONE 14(10): e0223995. https://doi.org/10.1371/journal.pone.0223995

[3] Anderson, H., Honkanen, L., Ruotanen, P. et al. Comprehensive genetic testing combined with citizen science reveals a recently characterized ancient MC1R mutation associated with partial recessive red phenotypes in dog. Canine Genet Epidemiol 7, 16 (2020). https://doi.org/10.1186/s40575-020-00095-7