In the world of horse breeding, the fascination with genetics runs deep. Understanding the genetic makeup of a horse is crucial for breeders aiming to produce offspring with specific traits – whether that be a horse with a flatter front-end, a deeper hock, or one that can get regular registry papers with the APHA. Two essential terms in equine genetics are "heterozygous" and "homozygous" – having a handle on these will allow you to understand how to seek out the stallions that will produce what you're looking for. In this article we'll be focusing on stock horse stallions and how to understand what a candidate offers genetically.
The Genetic Alphabet
Just like in human genetics, the genetic code in horses consists of DNA, the building blocks of 'everything'. DNA is composed of nucleotides, and the sequence of these nucleotides forms genes. Genes, in turn, determine a horse's traits, ranging from coat colour to performance abilities, and so much more.
In stock horses, as in many other breeds, some genes have more than one variant, known as an "allele." These alleles can be dominant or recessive, and understanding their interaction is crucial for predicting the traits that your foal may inherit.
Homozygous vs. Heterozygous: Defining the Fundamentals
Homozygous: When a horse carries two identical alleles for a particular gene, it is referred to as homozygous for that trait. For example, this means that if the stallion you've chosen is homozygous for the tobiano gene (TO/TO), all of his foals will exhibit the tobiano coat pattern and be heterozygous themselves (n/TO), regardless if the mare is solid. To add a layer of complexity if that same stallion who is homozygous for Tobiano is paired with a mare who is heterozygous for tobiano (n/TO), any resulting foal will 100% be tobiano 100%, but has the added benefit of 50% chance of being homozygous (TO/TO) themselves.
Heterozygous: Heterozygous individuals carry two different alleles for a particular gene, if we use the same example with a tobiano sire, this stallion being heterozygous (n/TO). This sire has a 50% chance of passing on the Tobiano gene; if he were paired with a mare that is also heterozygous, they both have a chance of passing on the tobiano gene and could produce both heterozygous (n/TO) and homozygous (TO/TO) foals.
Food for Thought
There are obviously any number of complexities associated with these terms as well, because a horse that is technically a tobiano via genetics may not express that tobianoism to the degree that is desired by most – perhaps he just has a stripe of white over his hips, or high white stockings but is not a loudly coloured tobiano. There's also a chance that the tobiano is expressed so minimally that the resulting foal wouldn't be eligible for regular registry within the APHA.
Stallion Examples
Count the Minutes (AQHA / APHA)
2010 Bay Overo (AQHA / APHA) Stallion, standing 16 hands, owned by Black Top Ranch; View his page on StallionCompare.
OLWS Positive (n/OLWS): He is heterozygous and carries a single allele for OLWS (Overo Lethal White Syndrome, which "is a genetic disorder that results from two copies of the version of the gene causing the frame overo coat color pattern," (UC Davis)). Two copies of OLWS (OLWS/OLWS) results in death of the foal. Any mare that is bred to him should be tested and negative for OLWS (found to be N/N on testing, meaning cannot also be heterozygous herself).
W20 Carrier (n/W20): W20 is a dominant white gene, he carries one copy and has a 50% chance of passing that W20 gene on to his foals.1
Homozygous Black: This is an interesting one and often misguides people – homozygous black means that this stallion will never produce a red or sorrel foal. He will not always produce black, but he will always produce black OR bay foals.
Fully Furnished
2021 Cremello (AQHA) Stallion, standing 15.1 hands, owned by Sam Korna; View his page on StallionCompare.
Homozygous Cream Gene: Fully Furnished is homozygous (Cr/Cr) and will transmit at least one Cream Gene (Heterozygous: n/Cr) to 100% of his foals. This means that regardless of the colour of the mare he is paired with, he will produce a single dilute variation of that base colour. We'll go into understanding the mechanics and variations of the cream gene in more detail in another article.
Holiday White Carrier (n/W35): The rather new W35 gene, named "Holiday White" and discovered by Etalon Daignostics in April of 2023. This new dominant white (W) spotting variant is known for excessive white (white face, belly spots, leg markings, with or without roaning and/or white patches ranging in expression). He carries a single allele for W35 and is therefore heterozygous for the gene, passing it on to 50% of foals. A stallion homozygous for Holiday White would be W35/W35.
MYHM Positive: He is heterozygous (n/MYHM) for the MYHM (Myosin-heavy chain myopathy) allele. This allele is associated with an increase in susceptibility to an autoimmune "muscle disease in Quarter Horses and related breeds that results in two distinct clinical disease presentations, immune-mediated myositis (IMM) and non-exertional rhabdomyolysis. Both presentations involve muscle loss or damage and are linked to the same genetic variant," (UC Davis). We'll cover more about MYHM and the implications of this disease in a future article.
Harnessing the power of equine genetics will assist any breeder and horse person in understanding the makeup of their horses, and we hope that information like this helps in making educated breeding decisions. Stay tuned for more explorations into the captivating realm of equine genetics and its impact on the world of stock horse breeding.
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References
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W20 (c.2045G>A) is a much older mutation and is found in many breeds. This mutation is thought to have a more minor effect on protein function as well as a subtler effect on the amount of white expressed unless in combination with other dominant white alleles (and perhaps other white spotting genes). (UC Davis)
Natashya V.
Founder, StallionCompare.com | Creative Director
Natashya, the founder of StallionCompare.com, is a dynamic force in both the equine and creative industries. As a lifelong equestrian, Natashya’s journey with horses began with Quarter Horses in her hometown in British Columbia, Canada. During her college years she took a brief (and eye opening) hiatus from Quarter Horses to work in the Thoroughbred industry – returning to the AQHA circuit as a young adult. Natashya was also able to reconnect with the quarter horse industry through design. She worked with former owners of Pleasurehorse.com and Show Horse Today on their design needs prior to Robyn Duplisea purchasing the business. Natashya also worked at Horse Canada (HPG) as the Creative Director for many monthly, quarterly, and annual equine magazines. Her passion (read: obsession) for breeding, genetics, and equine pedigrees ran ever-increasingly deep, which lead her to create StallionCompare.com.
Beyond her equine pursuits, Natashya is the Creative Director & Co-Founder of a leading design agency known for its boutique, customer-focused approach. With her expertise, she blends creativity and technology to make a lasting impact in both horse breeding and design.
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