The Agouti Series
The agouti series consists of:
Ay - sable
aw - agouti
at - tan points
a - recessive black
Broadly speaking, the agouti series controls which cells produce eumelanin (black pigment) and, in the case of the agouti gene itself, when.
This pattern of the spread of black hairs is followed by all of the genes in the A series (with the exception of recessive black). There is something inherent in the agouti series which causes this pattern, but as it's not found in most other animals with agouti, no one is entirely sure what it is.
It is important to note that the genes on the A series can only be expressed if the dog has one of the following genotypes on the K locus: kk, kkbr or kbrkbr. If the dog has just one K allele then it will be solid black (or liver/blue/isabella). A solid black dog may be genetically a sable, tan-point or agouti, but it will not be able to display it.
Sable (Ay) is the top dominant in the agouti series, so a dog only needs one sable gene to express it.
There are three types of pattern that can be caused by this gene, but it's not certain what causes each one to appear. It's probable that sable is simply affected by (as yet unidentified) modifiers.
The three patterns are: clear sable, tipped sable and shaded sable. The Shetland Sheepdog above is an example of a shaded sable.
Clear sables are completely red dogs with just a few black hairs. They can be almost impossible to distinguish from recessive red dogs (see the E series page) unless they have a black mask (Em), which never appears on recessive reds (because they're unable to produce any black hairs). If there is any black in the coat at all, the dog must be a sable rather than a recessive red.
Tipped sables are red dogs with black hairs, usually on the back, head, ears and tail. It seems that most tipped sables also have black masks (Em), so
it's possible there is a link between the two genes (although not all masked sables have tipping).
Shaded sables overlap with tipped sables and are red dogs with brown and black hairs covering the top of the head, ears and back, in a distinctive pattern similar to the pattern seen on dogs with creeping tan (see below). The shading can be very light (just some scattered dark hairs forming a rough pattern), or very dark and distinct. A distinctive feature is the "widow's peak" on the forehead, where the brown or black forms a point.
Grizzle in Salukis and domino in Afghan hounds are very similar patterns to shaded sables, but they have been shown to be caused by a different gene found only in those breeds. See the E series page for more information on grizzle/domino.
Agouti / Wolf Grey
Agouti is one of the oldest and most widespread mammal colour genes. It can be found on rodents, deer, wild rabbits, and it's even the main colour phase of the wolf. The main reason it's so popular is the camouflage it provides.
Agouti is typified by strands of fur which are banded. This means that as the fur is growing, first the cells produce one type of pigment (usually eumelanin, so black pigment) and then they switch to another type (usually phaeomelanin, so red). The gene that tells the cells to keep changing the pigment they produce is generally classified as aw.
Now the agouti gene in rabbits and deer produces banded hairs all over the coat, but for some reason the agouti gene in dogs is a little different. It restricts the banded hairs to certain parts of the coat, which are, roughly, the upper parts (back, head, top of legs etc). It's a gene inherited from the wolf, and can be seen on many breeds which are considered to be close to wolves (e.g. Czech Wolfdogs, Saarloos Wolfhonds), as well as some Northern spitz-type breeds (Keeshond, Elkhound).
In its normal form, agouti can be almost identical to shaded sable. The main difference is the banded hairs, but if you can't get close enough to see those, the pattern is also slightly different. Agouti tends to follow the same pattern as traditional tan points rather than creeping tan (see below), although it does appear to vary.
When combined with phaeomelanin dilution (see the C and I locus page), agouti becomes much more distinctive, as the red hairs become cream or greyish. The result is a wolf grey dog, like the Keeshond, Vallhund or Elkhound. The Elkhound below
has partial dilution of phaeomelanin, which leaves cream points, and the Husky has almost full dilution of phaeomelanin, leaving off-white points. The three dogs below are probable agoutis, although as stated above it can be difficult to tell these apart from shaded sables.
It's also worth noting that so-called "sable" German Shepherd Dogs are in fact agouti.
It is not clear whether this pattern in Akitas is due to the sable gene or agouti. The tipped/banded hairs concentrated along the back are reminiscent of agouti, although Akitas appear to lack the typical pattern of light points associated with the agouti gene, and also lack the "widow's peak" pattern of shaded sables.
Currently it is also unknown whether this colour in wirehaired Dachshunds is sable or agouti. However, the pattern appears to be much closer to agouti than sable.
Finally, one more mystery. Is this Shiba Inu agouti or sable? This one is most likely to be a sable as there is a distinctive widow's peak with the shading not continuing under the eyes and down the top of the muzzle as it does on the Husky in the section above, as well as the wirehaired Dachshund directly above.
The tan point gene (known as "traditional tan points") is almost the bottom recessive in the agouti series (the only one below it is recessive black, which is very rare). This means that, generally, a dog must have two copies of the tan point gene in order to express tan points, so their A locus genotype must be atat.
The range of markings on a tan pointed dog are very restricted. Red (tan) appears as pips above the eyes, on the sides of the muzzle extending to the cheeks, as pips on the cheeks, on the neck just below the head, as two triangular patches on the front of the chest, on the lower legs (and inside of the legs), and as a patch underneath the tail (and sometimes along the bottom edge of the tail too). The main colour is solid black (or any other eumelanin colour - liver, isabella or blue - depending on the other genes involved). Sometimes black marks are present on the toes. This is called "pencilling".
The Rottweiler and Dobermann are two of the most well-known breeds that carry only the tan-point allele on the A locus. All dogs of these breeds are tan-pointed.
These photos show just a few of the modifiers than can affect tan-pointed dogs. The Dobermann is homozygous for liver (bb), turning its black into brown. The Saluki cross in the middle has very light tan points, most likely modified by the Intensity gene that determines how rich the red (phaeomelanin) pigment is. The Cardigan Welsh Corgi has the brindle allele (kbr). Brindle only affects phaeomelanin, so this dog's points are brindle but its body remains solid black. This Corgi also has white in the irish spotting pattern.
Both of these Australian Shepherds have tan points and white in the irish spotting pattern. The dog on the left would be described as a "black tricolour" (or "black tri"). The dog on the right is genetically a black tri but also has the merle gene (Mm), which has diluted random sections of the black to grey.
Sometimes the points on the face can be covered up by a mask, as on these two dogs. The eyebrow pips are still visible on the Tibetan Mastiff, but the sides of the muzzle are covered by black. The German Shepherd crossbreed has a more extensive mask that also covers her eyebrow pips and partly obscures the points on the front of her chest.
Saddle Pattern and Creeping Tan
Previously, it was thought that the saddle and creeping tan patterns may be caused by their own alleles on the A locus. However, it is now thought that they are in fact modifiers of the tan point gene.
Recently the grizzle/domino gene in Salukis and Afghan Hounds was found to be an allele on the E locus that modifies tan points - it is possible that saddle and creeping tan have a similar basis, although neither modifier has yet been located.
The saddle/creeping tan modifier causes the black (or other eumelanin colour) on a black-and-tan dog to "retreat" to the dog's back, leaving the rest of the coat red. A dog with the creeping tan pattern has slightly more red/tan than a normal black-and-tan - usually spreading to the area around the eyes and extending further up the legs - see below for an example of this compared to a normal black-and-tan. The saddle pattern is the next step up from this, where the red extends over the whole head, the front of the chest/neck and the top of the legs, leaving black only on the back, tail and the back of the neck.
Sometimes the black saddle can have interspersed red hairs. This occurs in some terrier breeds and is known as grizzle.
Saddled and creeping tan dogs are usually born black-and-tan. The black recedes as the dog grows.
Compare this Airedale puppy with the adult on the right. The change is quite dramatic! Both of the photos were taken by Kenzie.
Photos of Revy submitted by Leah Petesch
This Pembroke Welsh Corgi shows black receding from 6 weeks to 3 years.
These three working Lakelands also show black "retreating". The dog on the left has the creeping tan pattern and the dog on the right has a large saddle. The dog inbetween would also be classed as creeping tan. Any one dog may go through all of these stages as it grows - or may just stay like the dog on the left.
The saddle pattern is the main colour of the German Shepherd Dog. This GSD also has a black muzzle because of the mask allele (Em).
Are these dogs with minimal saddles, or are they sables? The line between the two can be very blurred and it is not always possible to distinguish between them, even on dogs with more black. For example, look at the dogs in the shaded sable section above - are these actually shaded sables or do they have
creeping tan or saddles?
The German Pinscher below looks very similar to the dogs above, but is almost certainly a sable. We can be fairly sure of this because the German Pinscher does not generally come in any form of creeping or saddle tan. However, the two breeds shown above (English Foxhound and Border Terrier) do regularly come in saddle or creeping tan as well as sable, making it more difficult to distinguish between the colours.
The only way we can be certain that a dog has the saddle or creeping tan modifier is if the dog was born black-and-tan.
Recessive black is a rare, recently discovered gene which is the bottom recessive in the agouti series. See the page on black (K locus) for more information on it (it seemed to fit better there!).
Interactions With Other Locii
We have mostly been looking at the agouti locus alleles in isolation - i.e. without reference to other locii. This is necessary to get an idea of what the individual alleles actually do. However it may now be helpful to look at the bigger picture. No dog will "just" be affected by the agouti locus - all dogs have all the locii and their phenotype (the way they look) is affected by the combination of all of those locii.
When describing this Dobermann's phenotype we would say it is a simple black-and-tan, or, if we're being more precise, black with traditional tan points. This describes what the dog looks like but it doesn't give the full picture about its genotype (genetic makeup).
What are the genetic conditions that need to be fulfilled for this dog to have the black-and-tan phenotype? Let's look at the locii one by one.
A locus (agouti) - it must be atat (or theorectically ata, as recessive black (a) is the only allele more recessive than at).
B locus (liver) - it must be Bb or BB (bb would turn its eumelanin to brown).
C/I locus (phaeomelanin dilution) - it must have no phaeomelanin dilution (II?), as its tan is a rich colour.
D locus (dilution) - it must be Dd or DD (dd would turn its eumelanin to grey/blue).
E locus - it must be EE or Ee (if it was ee it would not be able to produce eumelanin, and as there is no mask it cannot have Em).
G locus (greying) - it must be gg as it has deep black (no greying).
H locus (harlequin) - it does not matter whether it is Hh, hh or HH as harlequin can only be expressed if the dog also has the merle allele.
K locus (dominant black and brindle) - it must be kk in order to even express its A locus alleles. It also cannot have a kbr allele because its tan is not brindled.
M locus (merle) - it must be mm (non-merle).
S locus (white spotting) - it must be SS (no white).
T locus (ticking) - it does not matter whether it is TT, Tt or tt as ticking can only be expressed if there are white areas.
No time to read the whole thing? Here's the quick version!
The distribution of phaeomelanin and eumelanin is controlled by the A (Agouti) locus. There are four alleles: Ay (sable), aw (agouti/wolf grey), at (tan points), and a (recessive black). The Agouti locus is only expressed when a dog does not have a dominant black (K) allele on the K locus, as dominant black overrides all of the agouti patterns.
Sable (Ay) is the top dominant. Sables can be clear (solid phaeomelanin), tipped or shaded (with eumelanin). They are often combined with a mask (Em), which may also increase the amount of shading present. Shading generally occurs on the back, top of the head and ears (with a distinctive "widow's peak"), tail, and sometimes the front of the chest.
Agouti/wolf grey (aw) causes banded hairs, mostly covering the back and head (the legs, chest and underside are usually clear phaeomelanin).
The tan point allele (at) causes phaeomelanin to be restricted to the legs, under the tail, front of the chest, muzzle and above the eyes. It is more recessive than sable or agouti, so most tan-pointed dogs are atat (technically they could be ata, but this is fairly rare).
The tan point pattern is sometimes modified to "saddle" or "creeping" tan. The exact modifier responsible is unknown but it may be on the E locus. Saddle and creeping tan dogs are born with the usual tan point pattern but the eumelanin retreats as the dog ages.
Recessive black is a rare allele only found in a handful of breeds. See the K locus page for more information.
Note that any of the Agouti patterns may come with any of the eumelanin pigment colours - black, blue, liver or isabella - as well as brindle, white spotting, merle, etc.