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Monday, June 4, 2012

Red and black...or How a purebred chestnut Friesian is possible.

Red and black...or How a purebred chestnut Friesian is possible.

by Dainelle Kinsel

{I know many of you may understand all of this completely--in that case, let this be a refresher course, and for those of you that this may be new for, please follow along! In all cases, enjoy the ride and let's see where it leads us.}
I'd like to discuss the two basic base colors found in the domestic horse, red and black. All horses are one shade or the other underneath modifiers. This article is going to consider the "norm" in horse colors, we will explore oddities and puzzles in later articles. 
    The color chestnut is produced by a pigment known as pheomelanin. It produces all of the coat colors that fall under chestnut, including black chestnut, sorrel, red chestnut, cherry chestnut, etc. Changes in point color come about through modifiers such as panagre, flaxen or sooty--and even some that have not yet been identified, but let's not canter before we have learned to walk.)
Incidentally, every single one of these colors is identical genetically. In these articles, I may use the terms "chestnut" and "red" interchangeably, but rarely will I use "sorrel." Now, knowing that all red horses are identical genetically, we can take that a step further. Phaemelanin is found on the extentsion locus on a horse's DNA. Now what? You say. Let me be more specific. A locus is given spot on a strand of DNA. It's a physical thing and I always seem to think of it like lego blocks. You know how you can stack a bunch of them together in different colors and the bands may be different widths? That's a pretty good illustration of one piece of DNA. Without getting too technical, let's take out one of those blocks: that's our locus. On that block let's pretend there's only two little circles sticking up. Each pair of genes fits right there. 
OK, are you still with me? 
Good! And you are awake, too--I must be doing something right!
Alright then, let's give our pigment's locus an abbreviation. In this case, it's going to be a lower-case "e." It is written out lower case to show that it is recessive, which means when paired with any other color (in the horse's case, black) it can hide. (Are we getting close to the Frieisan issue yet? you ask. Yep, bear with me a little longer.)
Now that we know red is abbreviated e, let me explain that every horse gets one half of its DNA from its sire and one half from its dam, which in turn gives us two slots on the locus. (Think about the lego block with just two little circles.)
In the case of a chestnut horse, each little circle has an "e" sticking to it. So, when we are discussing a chestnut horse, we can abbreviate his geneotype (what is on his DNA or the part we can't see with the naked eye) as "ee." This means he is carrying two copies of the red gene. (Also called alleles.)
Let's talk about black. Black pigment in mammals is known as eumelanin. It is also found on the horse's extension locus. It is abbreviated with a capital letter "E" to show that it is dominant over red. Now stop for a minute and understand this is really important! If a breed a black horse to a chestnut horse, more than likely the offspring will be black, especially if the horse is homozygous for E. Homo-means "two of the same" and refers to the fact that a black horse carrying two copies (or alleles) of black pigment, EE, cannot produces red pigment (phaemelanin.)
Got it? Great!
Now consider this--some black horses are not homozygous (carrying two of the same) for black, they are heterozygous (carrying two different copies), in which case it is abbreviated Ee. EE blacks and Ee blacks look exactly the same. Some may sunburn and fade to a reddish color, but they are not red, not at all.
Let's see if this makes sense: a horse with red pigment cannot produce black pigment and the opposite is true: a horse with black pigement cannot produce red pigment--uh oh! We forgot the exception to the rule!
A horse with black pigment cannot produce a horse with red pigment unless it is heterozygous! (Do you see where this is going yet?)
Ah ha! This is why purebred chestnut Friesians are possible, but rare. Apparently, several generations back Frieisans could be any color. At one point, the royal stud book decided they didn't like other colors (for whatever reason) and decreed that all purebred Friesians should be black. Fine, great, whatever.
For the most part, it worked because it seems that the majority of the horses were all EE (homozygous black.) Great. However, there were some who quietly remained Ee. They were still black on the outside, but carried both black and red pigment on the inside.
Somewhere, somehow, two of these horses were bred together. If that pairing created four foals, one would be EE, two would be Ee, and one would be ----ee!
So you see, with the advent of a small population and a small gene pool, it's actually a wonder this hasn't happened many times before. The chestnut horse is still a purebred, but the wrong color (it's the same problem that dogs professional canine breeders (sorry for the pun!) and we will discuss that at a later time.)
I haven't seen much information on whether Friesian breeders are actually DNA-typing their horses, but I hope they are, though, of course, if their $40,000 colt turns out to be Ee--are they going to post it on the internet?
What a great start. If you liked this article, stay tuned for next time when we discuss Agouti, the bay and brown baby maker.

1 comment:

  1. What a great article! I especially love the reference to Legos -- that is a great way to think about it for a beginner to color genetics.

    I myself was just explaining this concept to my boyfriend last week! (Not that he cared, of course!)