Understanding basic color inheritance in Dexters can seem complicated because two colors: black and red, are located on one gene so its a simple "either / or", but then there is the color Dun which is more like an "on / off" that "hides" the color black. So let's break the color options down into 9 simple principles.
Dexter cattle occur in three different colors: black, red, and dun. Two pairs of genes that are located on separate chromosomes control these colors. Cattle chromosome 8 contains the B (brown = Dexter dun aka TYRP1) locus Cattle chromosome 18 contains the E (extension = red aka MC1R) locus
Black and red is one pair of alternative colors. In Dexter cattle, there are two different red genes. The two red genes are alternatives to each other, and they are not visually distinguishable. However they are distinguishable by a DNA test. Both red genes are recessive to their black alternative. This means that every red Dexter contains two red genes, one inherited by its sire and one inherited by its dam.
Black and dun is another pair of alternative colors. The dun color in Dexters is due to a brown mutation, and it is recessive to its black alternative. This means that every dun Dexter contains two dun genes, one inherited by its sire and one inherited by its dam.
Two black Dexters can produce black, red, or dun calves. In order for two black parents to produce a red calf, each parent must carry a hidden red gene. In order for two black Dexters to produce a dun calf, each parent must carry a hidden dun gene.
In Dexter cattle, red is the only color that breeds true. Two red Dexters can produce only red calves even if each parent carries a hidden dun gene. A Dexter that carries two red genes and two dun genes is red in appearance.
If a red Dexter that has two dun genes is crossed with a dun Dexter that does not carry a red gene, the result will be a dun calf. Each dun calf resulting from this cross will carry a hidden red gene.
Two dun Dexters can produce dun or red calves. In order for two dun parents to produce a red calf, each parent must carry a hidden red gene. Every red calf that is produced by two dun parents will have two dun genes. (see #5 & #6 above)
When a red Dexter that does not carry a dun gene is crossed with a dun Dexter that does not carry a red gene, the result will be a black calf. Each black calf resulting from this cross will carry a hidden red gene and a hidden dun gene.
A non-black Dexter calf must be red if its sire or dam is known not to carry dun. Conversely, a non-black Dexter calf must be dun if its sire or dam is known not to carry red.
Additional genes, independent of the genes for the basic colors, may also be present in an animal’s genotype and may "modify" the appearance of the animal such as brindling, black noses in reds, black shading in reds, and the shade of color of reds and duns. Luckily, these modifier genes do not alter the principles contained in the preceding nine concepts so we will leave the topic of modifier genes for another post.