Our current monthly mystery horse is actually two horses this month: a Paint mare and her foal, both with unusual markings, but completely different to one another. The mare, Sparkle, is almost totally white with roany chestnut spots and blue eyes. This year she’s had a very loud colt, called Snow. One side of Snow is completely white, except for chestnut ears and forelock. The other side of him is 50:50 white. The sire of the colt is a chestnut frame overo (we haven’t tested him, but we’ve seen pictures, and it’s possible he carries more than just frame overo).
To identify their colours and patterns we tested for red/black, agouti, Appaloosa, frame overo, splashed white 1 (SW1), tobiano, sabino 1 and W20.
On the colour front, we found that both Sparkle and Snow were chestnut (e e, two copies of red pigment). Sparkle, the dam, was A a (one copy of the bay factor, and one copy non-bay) while Snow was A A (two copies of the bay factor).
The results of our pattern testing showed that Sparkle was positive for one copy of sabino 1 (SB1 n) and one copy of W20 (W20 n). She also carries one copy of SW1 (SW1 n). She was negative for all the other pattern genes that we tested: Appaloosa, frame overo, and tobiano.
Snow’s test results showed that he carries two copies of SW1 (SW1 SW1) and one copy of W20 (W20 n). Despite the large amount of white present and his sire being frame overo, Snow is negative for frame overo, tobiano and Appaloosa.
Both sabino 1 and W20 boost the expression of other white pattern genes. The effect of sabino 1 is quite strong, while W20 is a bit weaker. Sparkle is almost totally white because sabino 1 and W20 together cause a substantial amount of white, and the addition of SW1 takes that even further. It is also worth noting that sabino 1 and W20 are different variants of the same gene (called KIT), which means that Sparkle will always pass on either sabino 1 or W20 to her foals, never both and never neither.
Snow’s extensive white markings are from two copies of SW1, boosted by W20. The asymmetric nature of his markings and how extensive they are is most likely just part of the natural individual variation that we see with all coat colour genes.