A friend of mine pointed out that one of the reasons the term “black-based” is so popular is that the name of the gene is not particularly illuminating. She had a good point. Just why is it called Extension?
The other pigment-type switch in horses, Agouti, is a little more obvious. That locus takes its name from a South American rodent that is a close relative of the Guinea Pig. Because his early studies of coat color used Guinea Pigs, W. E. Castle gave the name to the color typical of wild rodents and to the factor he believed controlled the banding pattern that characterized it. It was usually called “A” or the “A Factor”. (The term “gene” had not yet been coined.) The piggies in the top image are examples of the agouti color.
Early in the course of these studies, it became clear that some other factor was responsible for the color in the lower picture, which is a self-colored red. The scientists noted that, unlike the albino Guinea Pigs, the red ones had black pigment in the skin and the eyes, but it was no longer present in the coat. Black pigment was Restricted to the skin, whereas in the agoutis, it Extended into the coat. You can still find the term Restricted (R) in some older writings on the subject, but the convention became that the “factor” (gene) took its name from the dominant (not the recessive) form. Extended was dominant to Restricted, so the factor (gene) became Extension.
So Extension was about whether black could extend from the skin into the coat. It never really was thought of as a “red or black” option so much as an “enabling black” option. Recessive reds (ee) were animals where black in the hair was not enabled, while those with the dominant allele (E) could have black pigment in the coat.
This is quite different from the way Extension tends to be seen in horses, which is as the choice between red and black. That explanation will work because horses have—at least at this point—only two options at Extension. That was not the case with the Guinea Pigs, and it is not the case with other species, some of which not only have other options at Extension, but more than just the two pigment-type switches. In the next few posts, I am going to veer off into some of those other pigment-type controls, so if you found the last few posts confusing, you might want to skip reading for a few days. But for those that have found the topic of pigment-type switching helpful, this will probably clarify why these are really a group of genes that all work together to give the final color. I’ll also try to tie this back to horses, and why I believe it is a good idea to lay this foundation now.
See the little guy to the right, with patches of red and agouti? He is a sneak preview for the next few posts, which will explore some of the alleles for Extension in other species, as well as other pigment-type switches.
And finally I want to thank everyone for the feedback they have given on these last few posts. I used to believe that the ideas behind horse color, if presented in just the right way, could give an immediate “aha!” response. That was how I judged whether or not an explanation was successful. As this subject has become increasingly complex in light of new discoveries, I have had to (quite reluctantly!) scale back my expectations and count it a success if an idea was clear to someone after a couple of readings. I really do appreciate the faith readers have that if they read a post a few times, I will eventually make sense. At a time when all of us are so very busy, the willingness to spend that kind of time on anyone’s written words is a profound compliment. I really will try not to make things any more confusing than necessary!
I will close with a quote from one of the earliest books about coat color inheritance. As far back as 1909, people looking at animal coloration noticed that any color was easier to understand than the original wild color of a species!
[the original coat] pigmentation, common to wild rabbits, is complex in its nature, and all other color varieties are relatively simpler. The [wild color] coat results from the joint action of several independent color factors; all other types of types of pigmentation result from a weakening or entire loss of one or other of the factors concerned in producing the [wild color]. — Studies of Inheritance in Rabbits, W. E. Castle
(Photo of the agouti Guinea Pigs by Tim Strater and courtesy of Wikimedia Commons, photo of the red Guinea Pig courtesy of Love-To-Share Stock Photography, photo of the agouti/red Guinea Pig by Carlosar and courtesy of Wikimedia Commons)