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|Author||Topic : Quick Colour Genetics Course - reposting|
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The American Cocker ShowDog Colour Genetic Model:
A quickie genetics course:
American Cocker colours in ShowDog are a bit complex. There are 6 loci involved - A, B, C, E, K, and S. The American Cocker has included the K Locus which makes it somewhat more complicated than the English Cocker. In reality the colours of the two breeds are pretty much the same although the frequency of occurrence of certain colours is different.
I will occasionally use some terms in a generic manner which may seem somewhat ambiguous, so to clarify, “dark” means black or brown, “solid” means without significant white – the opposite of “parti”, and “self” means one colour without tan-point or sable markings or shadings. A “self” coloured dog may or may not be parti-coloured – i.e. a black and white particolour is a “self” coloured parti; a black white and tan is a tan-pointed parti; a red and white is a “self” coloured parti; a red tri-colour is a (red) tan-pointed parti and a red bi-colour is a (red) tan-pointed solid.
The American Cocker model actually includes two different types of “self” colour, one of which is largely recessive and the other which is dominant. I will refer to these as “recessive-self” and “dominant-self”.
The term Locus (Loci is the plural form) simply refers to a particular spot in the DNA chain. The individual genes occur in pairs - one comes from the sire, the other from the dam. Depending on the locus there are (in the AC world of ShowDog) two or three possible forms of each gene - these forms are called "alleles".
Many, in fact most of the loci have interconnecting influences where the state of one locus will influence the expression of another locus, masking its effect or preventing its expression. This is where a multi-coloured breed gets very confusing. So, to prepare you for that there will be occasional notes referencing other loci which we have not yet discussed.
The Loci, their Alleles and Order of Dominance:
A – Ay, at, a
B – B, b
C – C, cch, ce
E - E, e
K - Kb, ky
S – S, sp
We use upper case to indicate the more dominant form. When I write "Ay/at", this says that this dog inherited the dominant “Ay” from one parent and the recessive “at” from the other parent.
The A Locus:
The A Locus determines the pattern of the dark pigment - i.e. sable or tan-points or self-coloured.
The possible combinations are “Ay/Ay”, “Ay/at”, “Ay/a”, “at/at”, “at/a” and “a/a”. “Ay” is the sable allele, “at” is the tan-point allele, and “a” is the recessive-self.
“Ay/Ay”, “Ay/at”, and “Ay/a” will all be sable.
“at/at” or “at/a” will be tan-points, including the bi or tri colours (red, buff, or silver).
“a/a” are recessive-self-coloured.
A dog that shows tan-points can not carry sable although it might carry recessive-self. A recessive-self can not carry either tan-points or sable.
NOTE - The actual physical expression of the A locus allele is dependant on both the E and K loci.
The E Locus:
The recessive e on the E Locus lacks the ability to produce dark pigment - black or brown. Therefore a dog that is e/e will be some shade of red-yellow (red, buff, or silver).
A tan-pointed (“at/at” or “at/a”) “e/e” is described as a bi-colour (or tri-colour if it is also a parti). In real life, the tan-point factor is expressed as a lighter shade in the pointed areas, usually described as lighter feathering. I suspect that it would be more obvious in the case of deeper reds and less obvious in paler silvers. In ShowDog we recognize this by labeling these as bi/tri colours. It merely supplies a little additional information as to the underlying genetics of the dog. We do not differentiate a sable or a self in this way – both conditions will be labeled red, buff, or silver, the shade depending on the C locus. On this basis, a red, buff, or silver may produce sable (or self) when bred to a “dark” tan-point dog when the “E” is added.
The K Locus:
The K locus affects how or if the alleles of the A Locus are expressed. The dominant Kb completely overrides the expression of the A Locus – “Kb/Kb” and “KB/ky” will all be self-coloured whether it is black, brown, red, buff, or silver. There will be no visible sable or tan-points.
All tan-point and sable dogs are pure recessive for “ky/ky” (the “y” is for yellow). With self-coloured dogs, it is impossible to distinguish which are dominant-self, due to the presence of “Kb”, and which are recessive-self due to “a/a” and “ky/ky”, and/or “e/e” except through breeding history.
The C Locus:
The C Locus determines the shade of red-yellow. Red (“C”) is dominant; silver (“ce”) is most recessive. “C/C”, “C/cch”, “C/ce” will all be red. “cch/cch” and “cch/ce” will be buff. “ce/ce” will be silver. This locus is only apparent with the recessive “e/e”. (In real life it will affect the tan-points and sable base shade as well as “brown”.)
The B Locus:
The B Locus determines the type of dark pigment. “B/B”, and “B/b” will be black - solid black, black and tan, black and white, black,tan and white. The recessive b/b will dilute the black to brown. “B/b” will be black but can produce brown.
This locus applies to both hair and skin pigment so that a “B/B” or “B/b” will have a black nose, eyerims, etc., while a “b/b” will have brown regardless of their coat colour or pattern. It is recognized in the ShowDog English Cocker descriptions but not the American. In American Cockers the status of this locus is only known in the presence of the dominant “E” with self-colours or tan-points. We are not told if a sable, red, buff, or silver has a brown nose and is thus “b/b”.
The S Locus:
The S Locus determines "White Spotting" - whether the dog is solid or particolour. S is the dominant allele; sp is the recessive and produces the particolour pattern. S/S, and S/sp will both be solid. sp/sp will be particoloured.
The coloured areas of a particolour are determined by the other loci. A “black, white and tan” is really a standard patterned black and tan with white. The appearance of white is totally independent of all the other genes.
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