A couple of definitions and a brief discussion:
In color genetics, we often want to think of genes as switches that are either off or on and separate from one another. This is good for explaining simple inheritance and gives people with no background in genetics an easy introduction. However, when you start to learn more complex subjects like linkage, you have to understand that this is an oversimplified approach. Gene are not simple, they are actually very complex. An average gene is made up of around 100,000 base pairs.
Calculate the possibilities of different colored offspring when the genotype of the parents is known or can be guessed.
White patterns in horses are known to cause bright blue eyes. The white pattern most often associated with blue eyes in horses is the splashed white pattern. KIT gene mutations (including Dominant White, Sabino, and Tobiano) are not as frequency associated with blue eyes but why? The reason lies with gene function and the distribution of pigment during embryonic development.
The brindle color pattern does not often occur in horses. The only mutation in horses that reliably produces a brindle coat color is now known to be associated with skin and other health problems and to be lethal in male foals. All other known brindles in horses are the result of chimerism or other mechanisms that do not reliably reproduce. Chimerism occurs when twins fuse very early in development.
When discussing white patterns in horses it is never long before the word Overo is enters the conversation. But what does Overo really mean? To be blunt, the term Overo, only means “Any white pattern that isn't Tobiano”. This is a carryover from times past when distinguishing between white patterns was impossible for most breeders. A simple way to categorize white patterns was needed for registration.
The past several days have been very exciting in the world of Equine Color Genetics. The results from the first round of W20 testing has started to filter into facebook and the results are very popular. However, this has caused a problem. More than once over the past several days, I have found myself explaining why this white pattern (and all white patterns at the KIT locus) should know be referred to as “White Spotting” rather than Dominant White or Sabino.Then it dawned on me...Why can’t we just call them all Sabino? These white patterns, phenotypically, look like Sabino. Who cares if they are inherited as an Dominant, incomplete dominant or recessive for that matter.
Classic Roan is a white pattern that causes white hairs to be distributed throughout the coat but leaves the face and lower legs solid. It also causes characteristic inverted V shapes just above the knees and hocks. Classic Roan is inherited as a Simple Dominant. This means that if a horse is roan one of it’s parents must have been roan as well. However, like all white patterns the expression of Classic Roan can vary.