UNDERSTANDING EQUINE GENETICS

DNA is a long, linear molecule that contains coded instructions for all living cells. It is basically the blueprint of the cell, and everything the cell does is coded in the specific sequence of the DNA: which cells should grow and when, which cells should become muscle, which should become liver cells etc. All of the structural and functional proteins in a cell are encoded in the genes lying along chromosomes.

Horse DNA, just like in humans, dogs, and almost every other multicellular organism, is inherited from the parents through the sperm and egg. Foals resemble their parents simply because they were formed using DNA from each parent. While horses may have characteristics of each parent, they are never exactly like either parent. This is because approximately half of the foal DNA comes from the mare, and half comes from the stallion. Which pieces the offspring get is basically random, and each foal gets a different subset of the parents' DNA. Thus, siblings share the same parents, but they usually do not have exactly the same DNA, nor exact anatomy and physiology. This is vital to Thoroughbred genetics, because a cross that generates a stakes-level horse one year may yield a horse two years later that never even makes it to the track due entirely to the difference in genetic information between the siblings.

DNA Chart There are four different types of nucleotides in DNA which are represented as 'A', 'T', 'C' and 'G'. These four are all that's necessary to write a code that describes our entire body plan. Sounds too simple? Keep in mind that Morse Code uses only four symbols (dot, dash, short spaces and long spaces), and you could spell out entire encyclopedias of knowledge with that simple code! The Universal Genetic Code is the instruction manual that the cell uses to read the DNA sequence of a gene and produce a corresponding protein. Proteins are made of amino acids that are strung together in a chain.

So depending how the words that are spelled out using these four bases, the resulting message the DNA is generating can be quite different between siblings. Consider that the following sentence is a strand of DNA:

Go down the stairs and grab the hose

Simple enough to understand. But if we change even one letter, it becomes either a little harder to understand, or completely incomprehensible.

Go down the stairs and grab the nose

Go down the stairs and gray the hose



So from these three sentences, we see three completely different sets of directions. It's the same way with DNA in a cell. A change in a single letter can make a protein completely non-functional, thus significantly affecting the physiology of the individual. An example in humans is Sickle Cell anemia, which is caused by a single base pair change (out of more than 3.4 billion in each cell!), resulting in red blood cells that don't function properly leading to anemia, brain damage, and kidney failure.

In horses, the same language rules apply. A simple change in the DNA sequence can drastically alter the athleticism of a horse. We have currently identified polymorphisms (changes in the DNA) in genes that are directly involved in muscle twitch speed, energy production and potential athletic ability. One that seems to be very important is in a gene linked to the control of muscle contraction. We have identified a polymorphism in this gene that leads to the improper function of this gene product, resulting in horses that are not fast enough to ever make it to the race track. However, morphologically they are indistinguishable from their successful counterparts, who have a different sequence in this region of the gene which allows sufficient speed to compete in a Thoroughbred race. So, some horses have two copies (one from the sire, one from the dam) for the gene that gives rise to average or better speed, some have two copies of the slow allele (we've never found a double slow horse yet that ever made it to a race), and some will have one of each called heterozygotes. Heterozygous individuals appear to have as much success racing as do the horses that have two copies of the speed allele, however when the time comes to breed them, it will be vital for the owners to know that they possess one copy of the slow allele. In the event that this heterozygote is crossed with another heterozygote, unbeknownst to the breeders unless the horses have been tested, we know from basic genetics that there is a 25% chance that the resulting offspring will end up with two copies of the slow gene, and most likely will not have the musculature to ever race successfully.

This is not to say that heterozygous horses cannot be used for breeding purposes! What it says is that breeders who understand this phenomenon can plan their crosses more effectively to yield a higher percentage of successful offspring, thus driving up the stud fee or purchase price at sale.

Current breeding assessment techniques lack this level of information. It is only through genetic screening that potential problems such as these can be discovered.
We at ThoroughGen are constantly scanning the equine genome for polymorphisms that will give owners, trainers, and breeders that important edge. Feel free to contact us with any questions you may have about Thoroughbred genetics or how our screening is performed.


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