Agouti is actually the normal or wild state for a cavy. The ticked effect is produced by having a Black or Chocolate shaft to the hair, tipped with essentially Red, Buff or White and the very extreme end of the hair ticked yet again with the Black or Chocolate base colour. On the belly of this breed there is a similar situation, but the base colour is much thinner, the Red/Buff/White band is thicker, and the ticking of the base colour at the extreme end of the hair is absent, which yields an apparently solid colour. The gene that controls this appearance is documented as having three forms:
A - Normal Agouti.
ar - Ticked Belly Agouti (Solid Agouti). The apparently soild belly is replaced by a ticked appearance there as well.
a - Non-Agouti. All the Black (and related) Self varieties have this form of the gene. However, it is important to realise that the Reds (and related colours) are not necessarily aa at this locus. Despite the fact that the non-existence, red gene (e) is recessive to black (E), and further that non-agouti (a) is recessive to agouti (A), when a cavy is homozygous for the red gene (ee) pretty well all the black pigment disappears from the coat. Thus the reds can carry agouti factor unseen. Be warned, then, that a Red x Black cross may give agouti offspring in the first generation. It is not necessarily the case that the Red based colours do carry the agouti gene: it is just that you have no way of knowing, short of a breeding test.
Conversely, it must also be true that all the Agouti varieties are EE at the extension locus.
In regard to the different colours of the Agouti breed, these are produced by different combinations of the Black and C series genes, just as for the Self’s (though it must be said, that selective breeding for specific shades and the effect of the ticking means that the colours do not necessarily stand out as being what they really (genetically) are sometimes). The six recognised Agouti colours are Golden (Black/Red); Lemon (Black/Buff); Silver (Black/White); Chocolate (Chocolate/Red); Cream (Chocolate/Buff); Cinnamon (Choclate/White).
I have a theory about the Agouti gene that I have never seen explicitly written, but which I think is a fairly safe conclusion given the known facts. All Agouti breeds (other than the Golden) throw a ‘dilute’. These are basically Black or Chocolate type Self’s Agouti fanciers use these in their breeding programmes to stop their strain from becoming too light. These light Agouti’s are not strickly speaking lighter coloured – rather the band of ticking is wider which leads to a lighter, slightly more blotcy appearance. I believe that the lighter Agouti’s are in fact homozygous for the Agouti gene (AA) and the darker ones (the showable animals) are heterozygous (Aa). I believe that if you interbreed the darker ones you should get 50% darker Agouti’s (Aa), 25% dilutes (aa) and 25% lighter Agouti’s (AA). Interbreeding dilutes with the lighter ones show 100% darker Agouti’s. The only thorn in the side of this theory is the fact the Golden’s do not throw a dilute, yet they have very fine ticking anyway. However, this may be due to the fact that selective breeding in the Golden has enabled them to be sufficiently good without the need to employ dilute. This would be borne out by the fact the Golden Agouti Dutch generally have the grosser, lighter type ticking (Dutch being not selected so hard for colour due to the fact that breeders have the important markings to select for also).
Please note that it is not a good idea to interbreed these dilutes with genuine Self’s unless you have a very good reason. Further, it’s probably an even worse idea to use a genuine Self as an Agouti dilute (as genuine Agouti dilutes will be carrying various modifier genes that help with such things as feet ticking, eye circles, etc that the genuine Self may not have.
Black and related colours
B - Black. Produces normal colouring in the coat. Unless affected by other genes a BB individual will have dark eyes and dark skin.
b - Chocolate. Causes a modification to the normally black pigment granules of the coat producing a chocolate effect. A bb individual also normally has a slight ruby cast to the eye and much lighter skin. The presence of bb in Red cavies (and related colours) may also give warmth to their colour whereas BB individuals may have a relatively matt version of the same colour.
Red and related colours
E - The Extension gene. Allows black pigment to extend throughout the coat.
e - Non - Extension. Allows red pigment to be seen in the coat. Black pigmentation of the eye and skin, however, is not affected; and red (ee) cavies may yet betray whether they carry a Black or Chocolate base by the skin and eye colour. Also, in ee cavies, a small amount of the black pigment does still appear in the coat. Thus whether a red cavy carries black or chocolate can slightly affect coat colour too.
C Series(Incompletely dominant in many situations)
C - Intense colours. No dilution of either black or red. Black eyes.