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Sheltie Special

Genes known to be involved with coat colour determination in Shetland Sheepdogs

 

Agouti             Dilution             Merle              Slate Grey            Tweed             Piebald / White             Texture

Significant genetic diversity between different breeds of dogs is generally expected, but the degree of variation within specific breeds is also very high.  Many, now distinct, breeds have evolved through the selective (intentional or not!) breeding between other breeds - as is very apparent in the Sheltie.  I've tried to keep this simple (for my benefit as much as yours!) but there is a brief list of commonly used terms here.  All of the genes involved in coat colour actually contribute to that of the Sheltie, but other than the genes listed below,  the others rarely vary within the breed.

 

The Agouti locus

The Agouti gene (A) has been extensively studied in mice & recent studies have shown that the similarities between mice and other mammals is startling.  This gene provides the basis for all coat colours in Shetland Sheepdogs.   There are currently six forms (alleles) of the gene known, but only 3 of these (*) are observed in Shelties as shown below.  The alleles are listed in decreasing order of dominance:

Symbol

Allele Colour

*   As Solid Black
*   Ay Sable
     Ag Grey
      As Saddle
*   At Black & Tan
*   Aa Completely Eumelanic - no pattern

The most dominant allele is the solid Black colour.   Visually, this is apparent as a completely black dog, but may vary from liver - blue grey colour when combined with other genes.  This As allele is completely eumelanic, is the most common found in dogs & accounts for the majority of the black dogs bred.  There has been some debate over whether this allele is actually found at the Agouti locus, or whether it is located elsewhere in the genome, but the consensus appears that this allele - regardless of it's location, is dominant over the other Agouti alleles.  I have mentioned this allele as it is generally the most common ,and may be confused with Aa , but it does not appear to be involved with determination of coat colour in Shelties, where Sable is the most dominant allele.

The Sable allele, Ay produces a mainly phaeomelanic phenotype with eumelanic tips on some of the hairs (i.e. sable hairs with black tips).  The majority of the coat is constant in colour with most variation being at the ear tips, back & tail.  Coat colour ranges from very pale sable to very dark sable where most of the body hairs are tipped in black.   Darker sables tend to show a common facial pattern with a distinct area of dark colour above / around the eyes.

If the solid black allele is dominant, then whenever it is inherited (whether in a homozygous or heterozygous manner) it should be expressed, thus producing the solid black phenotype.  So what differentiates between a pale sable colour and the shaded sable colour?  I do not know of a definitive explanation, but it is plausible that the lighter dogs are homozygous for Ay while the darker sables are heterozygous for Ay and At.  This possibility accounts for the varying degrees of the shaded sable colour as well as accounting for the dark regions on a 'lighter' Sheltie.  The Sable allele exerts more effect due to it's increased dominance, while the Black and Tan allele produces a varying effect due to incomplete dominance of the Sable allele. 

The Black & Tan allele is very common in hounds and herding breeds.  The body coat is black while there are distinct tan areas over each eye, on the sides of the chest, underside, on lower legs and around the rear end.  I have only seen B & T Shelties in books, but please remember that this allele represents Tricolours - I just haven't reached the contributing white factors yet! 

The Aallele for no patterned areas is more rare in dogs.  In the homozygous state, Aa Aa, the coat is entirely Black which may lead to confusion with the dominant Black allele.   Although more rarely noted, this allele is observed in Shelties.  Again, I have not yet dealt with the white regions so this allele should be present in it's homozygous state in Bicolours - Black & Whites as well as Bi-Blues.   We know that must be the allele present as Bicolours show recessive inheritance - only when Aa is present in the homozygous state are Bicolours observed...in the heterozygous state, they appear as Tricolours or Shaded Sables.

So....as far as the Agouti locus is concerned, the alleles are present as shown below:

Pure for Sable = two Ay alleles
Shaded Sable = one Ay and one At allele or one Ay and one Aa allele
Tricolour / Blue Merle = two At alleles or one At and one Aa allele
Bicolour (B & W / Blue Merle) = two Aa alleles

Following on from the basic coat pattern (as determined by the Agouti locus), other genes must now be explained.  All of these other genes act to modify the basic coat colour, mainly by dilution effects, but please bear in mind that the basic colour is always the underlying / modified feature.

 

The Dilution locus

The dilution gene mutation (D) is recessive and while double dilutes are fairly common in Beardies / Poodles, they are rare in Shelties.   Some literature may refer to this colour as, "Maltese Dilute".  There are only two alleles for this gene - a dominant, wild/type intense form and a recessive dilute form.  Double dilutes show reduced pigmentation, where black coat hairs appear unifromly blue-grey in colour, with the eyes and skin also becoming lighter in colour.   The coat of double dilutes often appears to have a gray / metallic sheen and nose colour may also be blue-ish.

Please note that the term 'double dilute' does actually refer to the recessive, dilute mutant allele being present in it's homozygous state, and not a 'double merle' which involves a completely different gene!

 

The Merle locus

The Merle colouring actually comes from a dominant mutation to the Merle gene (M).  It causes black regions to become pale with fully intense patches scattered throughout the rest of the coat (Burns & Fraser, 1966).  Tan may or may not be present in Blue Merles (absence known as Bi-Blues and explained above). There are several ideas as to the reason for both sides of the merle appearing different......but the merle allele does not affect tan areas.   As the mutant merle allele acts over the basic coat colour (at Agouti locus) any colour of Sheltie may show the merle effect.  Hence, double merles can arise from double sable merle matings as well as blue merles.  Given that I live in Northern Ireland, I would consider the 4 current colours of Shetland Sheepdogs to be Sable, Tricolour, Blue Merle and Bicolour.  I have only ever seen one sable merle (neither myself nor Dad can remember any others - & he has been showing shelties locally for 30 years), but I've mentioned it here as I believe they are fairly common outside the UK.  I'm hoping someone call tell me if the merle effect on sable merle is an all-over effect, or is it patched as in the blues?  Please email and let me know folks.  (My thanks to those people who have already contacted me, and I am always interested in different accounts.)

The Merle colour arises from a dominant mutation and should therefore be expressed in all carriers.  If a double merle is mated to anything (within reason!), then all offspring will carry the merle mutation and hence show merle colouring.  But.....now it gets really interesting:  The merle mutation (merle colour) is due to a transposable (moveable) element in the genome.  It is documented, especially by Sponenberg in1984, that there is a reversion rate on the merle mutation of 3-4% in germ cells - eggs / sperm.  Reversions are stable so there is no change back to merle.

According to the recorded statistics, double merles are usually very pale (not necessarily white, but generally) with defective hearing.  Not deaf, but then dogs hearing is generally very acute and shelties are themselves very adaptive - I'm not convinced one would always know.  They generally also have visually defective, microphthalmic eyes - I don't have any statistics for this, but this particular disorder is known to be limited to homozygous (double) merles and occurs only in breeds where merle is a colour.   Having said that, many of the 'old' Sheltie books claim that double merles mated to Tricolours produce spectacularly marked blue merle puppies.....then again, we bred a beautifully marked Sheltie bitch a few years ago.  Not the best example of the breed, but her markings were among the best I have seen - incidentally from a Blue Merle x Black & White mating.

Cryptic Merle is a colour I have only seen in books, but I guess there are a few out there.  These dogs are genetically blue, but do not obviously show the merle pattern.  Blue spots / hairs commonly appear on the tail or around the head, but even a single blue hair is sufficient to identify this colour as a merle.

Merle pigmentation ranges from pale silver to dark gray with varying regions of black patches.  One variant in merle colouring is dependant on another gene - dominant form of which is called 'Tweed' (Tw).  This variant is relatively rare and apparently only acts in the presence of the merle gene, causing black patches to be larger than 'normal'.  It also has the effect of causing more patches to be of intermediate colour between the light merle background and fully intense black patches (Sponenberg & Lamoreux, 1985). 

There have been reports of a very rare 'Slate Grey (Sg)' gene in Collie and other herding breeds.  This gene leads to a dilute phenotype and was identified / described by Ford in 1969.  It is in a completely different position to either the dilution or merle genes.  The black coat colour is lightened to a dark grey with the eyes generally lightening to greenish colour.  The dogs are difficult to distinguish from their littermates at birth, but become more obviously pale with age (Sponenberg & Rothschild, 2001).  All reports stress this is a rare variant, but it is well documented.  I guess there are a lot of enthusiasts on the web - does anyone know of any cases?

 

The Piebald locus

The Piebald gene (S) is the only gene characterised for different white patterns in dogs.  There are 4 forms (alleles) documented at the piebald locus and I'd guess other genes modify white patterning but, as yet, these remain unknown - at least to me!  The Piebald effect creates patches of white areas in the basic coat colour in the order of dominance shown below:

Symbol

Allele Effect

S+ Wild Type / little white present
Si Irish Pattern
Sp Piebald
Sw Extreme Piebald

The manner of inheritance of white is little understood, but it is well accepted that white areas on the body will decrease in size / fade with age.   Small white patches present at birth may disappear by 12 weeks of age.   Generally, a dog homozygous for the Wild Type S+ S+ allele will show no white at all (Black & Tan Shelties) - but occasionally these dogs have small white patches on their feet and / or their chest...I'd guess due to unknown modifier effects to the S+ S+ genotype.  I've seen Shelties with very little white present, and that only found around their feet, so perhaps the S+ S+ genotype is still found in Shelties... but is modified so that a coat with complete absence of white is no longer an option?

Other than the wild type allele, there are 3 possible mutations (other alleles) that cause varying degrees of white in the coat.  The Irish allele, Si, (nothing to do with Ireland!) is dominant and produces minimal white on the feet, chest, facial blaze and white at the collar...but the depth of white will vary between individuals - again due to modifier effects.  The less dominant Piebald allele, Sp, is similar to the Irish pattern, with more pronounced white present.  The dogs will show more white on the legs, chest, larger blaze on the muzzle and topskull, with a consistently broad collar.  The last allele, extreme Piebald (Sw), shows white over the ears and around the tail regions.  Having said that, the level of white present in a coat is also determined by the level of spotting...the less dominant the Piebald allele present, then the more spotting is generally present (i.e. more white).   Too much spotting leads to what I would term 'mismarks' in Shelties.  White coated Shelties arising from excessive white spotting usually show dark facial features, and have none of the congenital defects associated with the double merle phenotype - I have yet to see this phenotype, but I've often heard the term Colour Headed White used to describe this type of colouring.

The above Piebald summary is intended to convey the impression that one gene accounts for white markings on the legs, face, collar and tail tip.  Other regions of the genome then modify the effect of whatever alleles are present, to produce white regions  to a greater extent on the main body of the coat.

When a particular characteristic is selected for, it is often found to be genetically linked to other characteristics.  The same set of genes proposed to determine white coat colour in dogs (NOT the merle factor, but white factor) have been linked to a predisposition to blindness (Coren, 2000).

 

Coat Type

There are at least 8 different genes that determine the length, texture and shape of coats - 2 of which have different forms apparent when influencing the coat in Shetland Sheepdogs. 

1) Long hair (L) is generally recessive to short hair which implies that most Shelties carry the recessive allele at this locus (for long hair) - this allele leads to longer hair on the body, ears and feathers (legs!), but has no effect on facial hair. 

2)    Wavy coats (Wa) are generally recessive to straight coats and are often associated with softer coats in Shelties - this accounts for the occasional wavy coat, but please note that this wave should be apparent throughout the coat and not confused with the slight wave often observed at the shoulders.

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Well, that was fun!   I have tried to explain the inheritance of coat colour pattern in Shelties my way - i.e. as simply as possible.   I actually learned a lot just by writing this page so I hope that others will find my summary useful.  One point that I have not commented on is the effect that various disease mutations may have on coat appearance....maybe another page someday!

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***  Further details on other loci are available on my Coat Colour Genetics page II.

***  Reference sites for all information can be found by browsing the genetics links section of my recommended dog links site.  They were correct when this information was compiled, but please ask if you would prefer the written references.   I also have most of the original papers / books to hand - the recently published information at least.  Please click here for refs.

 

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