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There has been more selection for variation in the dog than almost any other domestic animal, these differences include size, body shape, coat type (smooth, rough, long, short) and colour. The type of coat a dog has is usually associated with the work they were originally bred to do, though some have been altered over the years for a more 'showy' look in the world of dog showing. The colour of the coat has also in many cases evolved due to the work the dog was intended for, usually to either be or not be easily seen against the working background.
In the Australian Cattle Dog the two accepted coat colours are 'red', properly known as 'sable', and 'blue', properly known as 'black and tan' or 'tanpoint'. There are other colour variations that appear from time to time but these two are the ones breeders would hope for in litters.
When you look at an ACD and see 'red' or 'blue and tan' this is the PHENOTYPE - what is on display. What you don't see but controls the colour of the dog is the GENOTYPE.
Every dog has two alleles for each gene that controls coat colour and a puppy inherits one copy of each these alleles from its sire and one copy from its dam.
When thinking about the 'genetic colour chart' for a dog only the first shown allele in each series is expressed. The other is carried. For example, black dog may be B/B or B/b (when not known it is shown as B_). If it is homozygous for B (B/B) then it cannot produce brown coloured offspring even if mated to a brown dog. However if it is heterozygous B/b then it carries the allele for brown, and mated to another heterozygous B/b there is a 25% chance of brown puppies in a litter (over a large sample).
GENE SERIES
Agouti - is a protein that affects the amount of melanin in a growing hair and causes a banding effect. It is a term taken from the name of a South American rodent with this type of colouring, but a more commonly understood example would be that of a wild rabbit. The agouti gene has been mapped to dog chromosome 24. No breed of dog has all of the alleles it is impossible to state with absolute certainty what the order of dominance is, however ongoing work into coat colour seem to suggest that it is likely (in order of decreasing dominance):
a^w (wild) a^t (tanpoint) a (recessive black)
Those alleles that come into play in the ACD are shown
below - those that do not directly affect the ACD are not shown here. a^y (sable)
- this is also sometimes described as dominant yellow. Sable gives a red/yellow phenotype, but the hair tips
are black (caused by eumelanin). The extent of black tipping can vary
considerably from light sables (where the amount of tipping is minimal and
may disappear entirely in the adult coat) to darker sables (where there is a
greater degree of tipping and this may remain in the ACD's coat throughout it's
life - these 'smutty reds' are considered undesirable in the show-ring). More
black tipping than on average was believed to be associated with whether a dog
had a homozygous (a^y/a^y) or heterozyous (in ACDs a^y/a^t) genotype.
Sable is the common genotype of red Australian Cattle Dogs either in a
homozygous or heterozygous form. However, is now known that some, if not all, of
the differences in darkness of fawn or sable is caused by another gene. a^s (saddle tan). Eumelanin is restricted to the back and side regions like a saddle (this pattern is seen in Airedales and German Shepherds) This is undesirable in the ACD. The puppy will look like a tanpoint as a puppy but as the dog matures the amount of black 'creeps' back exposing the tan below. The amount of creeping varies - you can go here to see an example of a dog with only a saddle left http://picasaweb.google.com/LebensberufKennel/ArviEliNoblesseNemzi
There is a school of thought that a^s is a^t with modifiers which cause a reduction in the amount of eumelanin as the dog develops. Some ACD puppies will show early signs of creeping at the back of their ears, which will be red. Some will only ever display minimal creeping (tan further up the legs than normally found in a tanpoint and black eye patches may decrease giving the dog a tan eye patch in its place), whilst others will become predominantly tan with only a small saddle of black on their backs.
Extension - This is the Melanocyte Stimulating Hormone Receptor Gene (MSHr) or Melanocortin Receptor 1 gene (MC1R), a gene that has been mapped to dog chromosome 5. This gene has three alleles, the most common being E and e. These alleles control the extension of eumelanin over the dog's body. The dominant form, E, is normal extension and the dog will have some black or brown in its coat because of the production of eumelanin. The recessive form, e, is non-extension. When a dog is homozygous for non-extension (e/e), its coat will be entirely red/yellow (phaeomelanin based). The third allele at E is EM - known as melanistic mask. This allele allows agouti to bind some of the time and so fawn pigment will be made on the body, however the melanocyte stimulating hormone binds to the face. This means any fawn/red coloured dog must be so because of an agouti genotype and are never non-extension e/e (see below) as they require an EM allele to produce the black mask. Occasionally red Australian cattle dogs will have 'blue' muzzles, and even very occasionally black markings around the eyes, it is possible that this is caused by this third allele, but if so, it is rare.
The non-extension (e/e) dog does not have any black/brown hair in its coat and this helps differentiate it from an Agouti red. e/e ACDs have likely been cropping up in litters since the beginnings of the breed. When one parent is red then they will not cause any comment as red puppies would be expected in the litter. However, when both parents are blue then this is a sure indicator that both parents carry the recessive e and the puppy is homozygous e/e. The first DNA tested e/e reds were bred by Suzanne Nevada of Silveraurora ACDs. You can see photos and read about one of them here http://www.telusplanet.net/public/ranchrat/hyblade.html and pedigree can be seen here http://www.cattledog.com/cgi-bin/geneal.cgi?op=tree&index=38996&gens=5&db=kb.dbw
Another possible BBee ACD (not DNA tested) was Heli Sutinen's Windy (Cattlefarms Wild Wind) if you go to Windy's web page at http://www.kennel-lebensberuf.net/windy.html you can clearly see her pink/brown nose.
Recessive genes can be carried for many generations before the right (or some
would say wrong) combination comes along for the recessive to be expressed, if
it is a particularly rare recessive then it could be carried for many, many
generations. Brown, sometimes described as chocolate, ACDs are rare but not unknown.
D (Dilution) - ACDs are believed only have the dominant form of this 2 allele gene series, which is D and d. D/D produces undiluted black or red/yellow. Dogs that are d/d may be blue/charcoal gray as a dilution of black or pale brown as a dilution of brown and so on.
C (Albino) - The typical gene associated in other species with Tyrosinase. Controls the intensity of melanin in the coat. The dominant form C is described as 'full colour'.
Sponenberg and Rothschild describe a gene they named I for Intense that dilutes only phaeomelanin (red/yellow). Dogs with diluted phaeomelanin may be described as apricot, buff, cream or lemon. It is likely such a gene exists but has yet to be identified in the dog.
S - (White Spotting) - The first gene responsible for at least some of the spotting patterns has been identified and published in 2007. Potential mutations causing some forms of spotting have been identified. This gene is called MITF.
The
alleles currently at this series that affect the ACD are (in decreasing
dominance):-
The line drawn between a dog with few piebald spots and a dog that is extreme white spotted is not that clear-cut.
Australian Cattle Dogs can be homozygous piebald (s^p/s^p), homozygous extreme white spotted (s^w/s^w) or heterozygous (s^p/s^w) combined with the ticking gene.
T - (Ticking) - This is a two allele gene series where the incompletely dominant mutation T can only be expressed in areas that are white due to the White Spotting series (S). The colour of the ticking is the colour the dog would be if the white spotting gene had not been present (in the case of ACDs that would be black and tan or red). Dogs that are t/t will not be ticked. Ticking is not visible at birth, it develops over a period of time, hence the reason that ACD pups are predominantly white when born.
M -
(Merle) - ACDs are never merle but are often described as such, particularly
in those that are mottled. So at this locus ACDs are m/m
The order of dominance is shown to be: K^B - dominant black K^br - brindle k^y - normal
Brindle ACDs are, to the best of my knowledge, unknown therefore the two in this series that should concern us are K^B and k^y. Dogs which have two recessive alleles (k^y/k^y) can express a variety of phenotypes. All black-and-tan dogs or dogs with tan points are k^y/k^y. All fawn or sable dogs are k^y/k^y , whether they have a melanistic mask or not. Red dogs that have an e/e genotype however, could be any genotype at the K locus. The Stanford group chose to name this allele k^y since it allows yellow or phaeomelanin pigment to show. Where it shows depends on the alleles at the agouti locus.
A simple chart
Most new ACD breeders start off just wanting to know if they will have red or blue puppies. So, I will show some Punnett squares below that may help work this out. The assumption is that all the dogs involved in these hypothetical breedings are E/E so there will be no red puppies in blue matings. It doesn't matter which dog is sire and which dog is dam. I have coloured the sable allele red and the tanpoint allele blue which, hopefully, will help.
Mating 2 blue ACDs together (a^t/a^t x a^t/a^t):-
Both parents are a^t/a^t and can, therefore, only pass on a^t alleles to their offspring. All puppies will, therefore, be blue. Mating a blue ACD with a homozygous red (a^t/a^t x a^y/a^y):- A homozygous red does not carry the allele for blue
The red a^y allele over-rides the a^t allele, so all puppies will be red and will carry the allele for blue.
Mating a blue ACD with a heterozygous red (a^t/a^t x a^y/a^t):-
The heterozygous red carries the allele for blue.
In such a mating you could expect, over a large sample, to have 50% red puppies carrying blue and 50% blue. So, from this mating both reds and blues are possible.
Mating a heterozygous red (carries blue) and a homozygous red (does not carry blue) (a^y/a^t x a^y/a^y):-
All puppies will be red and over a large sample 50% will carry blue whilst 50% will not.
Mating 2 heterozygous reds together (a^y/a^t x a^y/a^t):-
Over a large sample 75% of puppies would be red,of these 25% would be homozygous reds and 50% heterozygous reds. The other 25% of puppies would be blue.
Mating 2 homozygous reds together (a^y/a^y x a^y/a^y):-
The homozygous reds only have a^y alleles so can only pass them on, therefore, all puppies will be homozygous reds too.
Contact Norma Digby on 01948 841302 (Shropshire, England, UK) |
