Unlike most animals, black in quarter horses is a recessive gene. Yes, recessive. A true black is a/a. Many horses that look black actually carry an Agouti gene, meaning a modifier on the black gene. A bay horse carries two Agouti genes so genetically is A/A. As the big A is dominant, it is much easier to get a bay than it is to get a black. To get a black foal, you must have two parents that carry the recessive a. The only way to guarantee a black foal is to breed two black parents, meaning both parents are a/a.
Once you have got the a/a, to get a grulla, the foal then needs to carry a modifier. To be a grulla, the horse must carry the basic body color of a/a, then carry at least one dun gene. The dun gene is represented by D. Because the modifier dillutes the basic black body and puts the dun markings on the black body, the resulting color is grulla. Dun is a dominant dulution gene, so even with one D, the horse is grulla. A grulla can be D/D meaning it is homozygous for dun, but that does not change the look of the horse.
Below are three foals that have been born at KT Ranch in 2016. It is an interesting contrast to look at all three colts, how they have tested genetically, and to look at their parents.
The fist foal is out of a grulla mare and a dunskin roan stud (Ladybug (Ladybug is shown above) and Gunner). He has color tested as a grulla- a/a, D/d, E/e. As an additional modifier he also carries the roan gene. Because the roan gene is dominant, this gene is also expressed and is represented by R/r.
The second foal is a foal that many people think is a grulla. He has a very silver color and a dominant dorsal stripe and shoulder barring. He is out of two black parents, neither of which carry the dun gene. Approximately 8 weeks ago UC Davis developed a test for the primitive marking gene which looks like dun. This colt has tested as a black colt, a/a. He does not carry the dun gene so will look black when he sheds out and is his mature color. The primitive markings should fade.
The third foal is very interesting. He is out of a chestnut stud that carries a black gene, and a black pointed dun mare (Double and Whinny). At first glance, most people would not think he could be grulla with a red parent. The key to remember is that a chestnut horse can carry a black body gene. What the sire won’t carry is an extension gene, or a black mane and tail gene. In this colt, the black mane and tail come from the dam, as does the dun gene. This colt color tested as a true black body, a/a. He clearly carries a dun gene, so he is in fact a grulla colt.
For more information on color, AQHA has a color expert, Lisa Covey, or go to UC Davis and look at their equine color website.