Kirk,
Here is an interesting animal for your color commentary.
I had a breeder mention this bull yesterday - and had never seen his photo so I said I would dig it out and send to him. I think many on this forum have not seen a brindled Dexter before.
Here's how it works (Mostly fact, with a little theory thrown in)
E (extention) Locus = primary controller of animal's ability to produce black pigment and/or red pigment
E+ = wild type = mix of red and black pigments
e = kill most all of the black pigment, leave mostly red behind
ED = overwhelm with black pigment
B (Brown) Locus = TYRP1 protein that helps any black pigment finish "cooking" (catalyzing) to actually look black. If an animal has two recessive "broken" genes at this locus, then any black pigment remains unfinished and looks brown (dun).
A (Agouti) Locus = A patterning gene that works with the E+ gene, to produce the typical wild-type shading patterns that we see in wild-type animals like the now extinct wild-ox. This locus is little-studied in cattle.
Br (Brindle) Locus = a locus that controls the brindling stripes of red and black... A BR dominant gene at that locus will give you a brindle cow, in an E+ cow that has lots of black AND red pigment.
The various genes at these primary 4 loci, likely control most all of the colors that we see in dexters (with other minor shading genes involved too).
The E locus, B locus, and Br locus are all three fairly well understood, but the A locus is not understood well in cattle. The old wild-ox likely was mostly E+E+ at the E locus, and aw/aw (wild-type agouti) at the A Locus... This gives reddish calves that turn quite dark with lots of black pigment (especially in bulls) as they age. aw is likely responsible for the standard pattern of black on red found in some of the darker E+ dexters.
Now it's very likely that the Agouti gene has multiple mutated variants. There is likely an Agouti variant that cancels out most of the black shading. So an E+ gene (without an ED gene) gives the animal the ability to make lots of both red and black pigment, but an Ax (agouti variant) may simply kill most of the black pigment... So the E+ animal with an Ax gene just looks red with very little black shading.
So, to get a true brindle, you definitely need an E+ (and no ED) gene (to allow for lots of red and black pigment), and you likely need two aw genes (instructing for lots of use of black pigment shading in certain areas), and you need at least one dominant BR (brindle) gene which causes any black pigment to be striped.
I believe its our lack of understanding of the Agouti genes in Dexters (and most cattle) that makes the various red and black shadings in E+ cows seem mysterious.
The brindle gene is as old as time and is likely found in every single breed of cattle. It likely even sprouts up as a fresh mutation from time to time. But because the brindle gene needs the correct combinations of Extension locus genes, and Agouti genes in order to exhibit, the brindle gene can hide very well and seem mysterious.
Brindle hides in blacks (although you might see a minor ghost pattern in some animals)
Brindle hides in true-reds (little e)
Brindle hides in many E+ reds (if they have certain agouti variants that cancel black pigment)
PS. Breeding multiple species really helps one to understand genetics. In our Icelandic sheep, they have a large number of colors and patterns and the Agouti gene has 6 variants, some of which can mix and match with each other. With a good understanding of the agouti genes then the patterns and colors are nearly 100% predictable. Without that understanding of Agouti, it's a nightmare to figure out.
I had a breeder mention this bull yesterday - and had never seen his photo so I said I would dig it out and send to him. I think many on this forum have not seen a brindled Dexter before.
