The Basics of Genetics

PHENOTYPE: This is what is on display, the effect you SEE when you look at a dog
GENOTYPE: This is what you don't see, but controls the color of the dog
LOCI / LOCUS: This is the gene's specific 'address' in the gene mapping, similar to your house's specific address on the street you live on.
ALLELES: This is the name of the variant form of a gene. Each gene can occur in more than one form.
GENE SERIES: The several alleles that may be present at a given locus are referred to as a gene series
HOMOZYGOUS: This means the allele pairing is the same, for example, B/B (only has the alleles for black).
HETEROZYGOUS: This means that the allele pairing is different, for example B/b (is black and carries brown).
DOMINANT: The allele that suppresses the others in a gene series is called the dominant allele.
RECESSIVE: The alleles that are suppressed by others are called recessive alleles.

Some factors of canine genetics:

Each locus is occupied by two alleles of each gene, one inherited from each parent.

Genes (and their alleles) are identified by letter names; the letter name may include a superscript, as in A^y.

In a gene series, the wild type allele is commonly (but not always) the allele that suppresses the effect of the other alleles.

Where more than two variants exist at a given locus, the alleles can be arranged in order of decreasing dominance.

Genes at some loci interact with the genes at other loci.

In the 'genetic color chart' for a dog only the first shown allele in each series is expressed, this is the dominant allele. The other is carried. For example, a black dog may be B/B or B/b (when not known whether the carried allele is B or b it is shown as B_). If the dog is homozygous for B (B/B) then it cannot produce brown colored offspring even if mated to a brown dog because both of its alleles are for black, and the black alleles, being dominant, will over-ride the brown making all the puppies black. 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) when the b/b combination comes together.

In the Australian Cattle Dog the two accepted coat colors are 'red' (properly known as 'sable'), and 'blue' (properly known as 'tanpoint'). There are other color variations that appear from time to time but these two are the ones breeders would hope for in litters.

In Australian Cattle Dogs sable is dominant to tanpoint (i.e. red is dominant to blue), so a red ACD can carry blue and if mated to a dog that is blue or a red carrying blue can produce blue puppies, but a blue ACD cannot carry the more dominant red allele, so two blues mated together cannot produce red pups except with the assistance of another allele (e), which does happen but is uncommon. In 99.9% of cases a blue to blue will produce blue.

GENE SERIES

A (agouti) - is a protein that affects the amount of melanin in a growing hair and causes a banding effect. The agouti gene has been mapped to dog chromosome 24. No breed of dog has all of the alleles and it is impossible to state with absolute certainty what the order of dominance is, however ongoing work into coat color seem to suggest that it is likely (in order of decreasing dominance):

A^y (sable)

a^w (wild)

a^s (saddle)

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 (A^y/a^t) genotype. Sable is the common genotype of red Australian Cattle Dogs either in a homozygous or heterozygous form.

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.

a^t (tanpoint) - This a dog that is colored, more often black or brown, on the dorsal area with tan (phaeomelanin) points found above the eyes and on the muzzle, cheeks, chest, throat, stomach, lower legs and under the tail. The points can range from creamy to mahogany in color. This coloration is found in many breeds, for example the Dobermann and Rottweiler. This is the typical color variety described in Australian Cattle Dogs as blue and tan, or tanpoint. Since red ACDs have an allele further up the chain of dominance blue ACDs are homozygous a^t/a^t and cannot carry red, though (an allele can only be carried by another that is more dominant, as shown below, two blues may produce red puppies from time to time.

E (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 E^m - 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 colored dog must be so because of an agouti genotype and are never non-extension e/e (see below) as they require an E^m 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 vast majority of ACDs are homozygous E/E

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.

B (brown) - this gene has no effect on red/yellow, only a lightening effect on eumelanin. The dominant form of this two allele gene series is B which programmes the color and pigment of the ACD to be black. In the homozygous recessive form (b/b) the pigment (nose, lips and eyerims) and color of the dog will be brown, there are now 3 common mutations recognised - b^s, b^d and b^c - and perhaps more rare ones that lead to brown instead of black eumelanin production. Whilst b/b has no effect on the A-series, it is possible to have an ACD that is e/e and also b/b, so the dog will be red/yellow with brown pigment. b/b dogs also have lighter (could be described as 'toffee colored') irises. 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 color'.

I (intensity) - 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 (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):

s^p (Piebald) - In a piebald dog the spots appear randomly anywhere on the body, they are not consistent in size or location, the white often crosses the back, which differentiates piebald from another allele known as Irish Spotting. Piebald is often combined with large markings on the body, sometimes described as 'slabbing' or 'plating'. A piebald dog often has color and white in an approximately 50/50 ratio. ACDs witha large amount of body spots are displaying the effects of the piebald allele.

s^w (Extreme White Spotted) - A dog that is homozygous for s^w/s^w will be mainly white, for example, a Sealyham terrier. ACDs with no colored markings, or markings confined to the head and, possibly a tailspot, when born are most likely homozygous at this allele.

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 color of the ticking is the color 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

K^B (dominant black) - It was long believed that solid colored dogs were at the top of the dominance chain for the A series and was known as A^s. However, Kerns et al. (2003, 2005) recently showed, through DNA studies, that this 'Solid' allele is not an agouti allele. They, instead, describe dominant black as a genotype that is epistatic to fawn, sable etc. and occurs at another locus which is now known as K^B (for blacK).

The order of dominance is shown to be:

K^B - dominant black

K^br - brindle

k^y - normal

Brindle ACDs are 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.