UNDERSTANDING COLOUR IN BULL TERRIERS
By Steven Liversedge -Stelron Bull Terriers
April 2005

The days of seeing only red/fawn or red brindle coloured Bull Terriers in the show ring is long gone. Now days, there is a significant increase in the number of tri-colours and black brindles combined with an increase in the number of coloured Bull Terriers displaying much more white on their bodies or white patches breaking the preferred colour markings. 

Not having bred much with tri-colours or black brindles in the past, it was also interesting to note that when mating a white carrying red brindle, to a red brindle and white (coloured dog), I produced a tri-colour. In the old days, I would have expected white, red/fawn brindle or red/fawn colouring in the pups. Clearly the tri-colour is a recessive colour and only displayed when the Bull Terrier carries no other colour.

As tri-colour is not my most favourite colour and based on the general consensus that red brindle is the preferred colour and that red or fawn are not the ideal colours to be carried by white Bull Terriers, I decided to do some research on the subject of “canine colour genetics”. Below, in layman’s terms, is what I have managed to gleam.

First of all, it is generally recognised that there are about 10 gene series that influence the colour variations we see. Some genes are dominant while others are recessive if included in the make of the dog.

The gene series that have the largest impact on Bull Terriers are as follows:

The agouti or “A’ series 
This series of genes defines the base colour of all Bull Terriers with it being recognised that there are basically three base colours in Bull Terriers, namely black produced by the As gene, tan (red) produced by the ay gene and tri-colour produced by the at gene in double dose. Note – capital ‘A’ means gene is dominant while small ‘a’ means gene is recessive. 

Knowing that a Bull Terrier inherits a gene from each parent, it must be recognised that various combination of the above are possible and the actual colour displayed will therefore be influenced by the dominant/ recessive factor, such as Asat (black in colour but carrying the recessive tri-colour), ayay (red in colour and pure for red), ayat (red in colour but carrying the recessive tri-colour) etc.

From a Bull Terrier perspective it was interesting to note the three base colours and how brindle is not considered a base colour. 

The white spotting or “S” series 
This series of genes defines the extent of white marking on a dog with the “S” gene influencing the extent of colour displayed while the sw gene is responsible for the extent of white piebald markings. Inheriting sw from both parents (swsw) would mean the dog is totally white while inheriting (Ssw) would mean a dog is coloured and white. The “S” series of genes also seems to be affected by modifiers, which can either decrease or increase the white displayed.

It was also interesting to note that other genes influenced head marking as well. 

The full colour or “C” series 
This series of genes prescribes to what extent the colour dictated by other gene series is expressed. In short it controls the degree of expression of the pigment in the coat.

The dilution or “D” series 
The “D” series controls how the pigment is clumped together, thereby influencing the shades of colour associated with Black, Red or Fawn. This series of genes can also have an effect on eye and skin colour. A dog inheriting “dd” from it parents would be more diluted in colour as opposed to “Dd” or “DD”. 

The brown or “B” series
The brown gene series controls the black pigment. A dog with the “B” gene in single or double dose would display black pigment, where as “bb” would display brown pigment. It is this series of genes that controls whether the dog has black, brown or liver pigmentation in the coat. Again this series of genes can influence skin pigmentation and the colour of the iris in the eye (light eyes).

The extension or “E” series
This series or genes are an extension in colour. Also know as the masking series of genes, the influence of this gene series is best recognised by the “Em” gene that causes the black mask on red smuts or the “Ebr” gene which masks the based colour with brindle stripping.

The brindle mask over a red base colour displays the desired red brindle, while the brindle mask over a black base colour would shows as a black colour, with no red or fawn visible. Brindle on a tri-colour base would show as black with only signs of the brindle showing in the tan (Red) areas of the tri-colour. 

The ticking or "T” series
This gene series is self-explanatory and affects the degree of ticking in a coat. Ticking if visible in the coat is a dominant gene. I am sure the degree of ticking in a white Bull Terrier is also influenced by the “S” series of genes and associated modifiers.

The important aspect to realise is that there are three colours in Bull Terriers, namely, black, tan (red) and tri-colour and that brindle is not a true colour but an extension of colour in the form of a mask over the base colour. 

A major concern today, is how breeders and judges are incorrectly classifying or interpreting two base colours as one colour, namely black brindle and tri-colour brindle. What breeders need to take note of is a tri-coloured brindle IS NOT a black brindle and more importantly a tri-coloured brindle is pure for tri-colour, meaning it does not carry the red or black base colour and can only pass on tri-colour in a breeding program. 

As breeders, a concerted effort is needed to safe guard the red and black base colours, as we are currently seeing the tri-colour through the mistaken use of tri-colour brindles, (thinking they are black brindles) become a much more prominent colour and being propagated at the expense of red and black. Just ask yourself, how often do you see tri-colour or tri-colour brindles in the ring today compared with reds, red brindles or the correct black brindles? I would hate to think what percentage of Bull Terriers currently classified as black brindle are in actual fact tri-colour brindle. I think we would all be staggered by the high percentage. 

Through intelligent discussion breeders should promote an understanding of the three base colours and the brindle factor. Understanding that tri-colour is a recessive colour and therefore Bull Terriers displaying the colour can only pass on the tri-colour (tri-colours, tri-colour brindles and whites carrying tri-colour or tri-colour brindle) means breeders should look to Bull Terriers carrying red, red brindle or the correct black brindle as complementary breeding partners. Likewise, as breeders look to safeguard the brindle factor, so should they safeguard the red and black base colours. 

The above indicates that the base colours are not the factors influencing eye colour, skin pigmentation, ticking etc. These factors are influence by the gene series associated with coat colour dilution and skin pigmentation. For me, the maintaining of a deep rich red colour is as important as the brindle factor.