06/07/2025
Very interesting read
Hey fellow horse genetics nerds—this one’s for us.
You know that spark of excitement when someone drops “mitochondrial DNA” into a casual conversation, And if you’ve ever bred, owned, or admired crosses involving Friesians, Gypsy Vanners, Clydesdales, or Shires, then you’ve probably seen something curious that doesn't quite line up with a basic Punnett square.
Why can two crosses involving the same breeds produce totally different-looking foals—just based on which parent is the sire and which is the dam?
For example, a Friesian mare bred to a Gypsy stallion will sometimes give you a compact, baroque-type foal—possibly moderate or heavy in feather, and usually with strong Friesian presence. But flip the cross—use a Gypsy mare with a Friesian stallion—and suddenly the foal might be leggier, taller, maybe more refined, and sometimes more fluid in movement.
This isn’t just anecdotal. There’s real science behind it—fascinating layers of maternal effects, mitochondrial inheritance, genomic imprinting, polygenic traits all working together to shape the outcome.
And to drive this point home, one of the clearest examples doesn’t even come from horse × horse crosses. It comes from a classic hybrid we all know: the mule vs. hinny. Breed a donkey jack to a mare, and you get a mule—a strong, hardy, generally larger hybrid with distinct donkey features. Reverse the cross—breed a horse stallion to a donkey jenny—and you get a hinny, often smaller, finer, and more horse-like in disposition. Even though both are half horse, half donkey, the difference in which species is the dam creates a vastly different result. They’re so distinct, in fact, that they have separate names.
Horses may not have the same interspecies boundary, but the direction of the cross still plays a huge role in how traits are expressed.
The mare’s contribution to her foal goes far beyond her 50% genetic share. She provides the entire developmental environment—including the uterus, placenta, hormones, nutrition, and mitochondrial DNA. These maternal effects can influence:
*Overall size and fetal growth rate
*Limb length and joint angles
*Bone density and body width
*Muscle development and metabolism
*Early temperament and reactivity
The most overlooked factor here is mitochondrial DNA (mtDNA), which controls cellular energy production and is only passed down from the dam. That means the foal’s metabolism, endurance, and even muscling tendencies are influenced by the mother—regardless of what the sire contributes.
So, when you breed a Shire mare to a Friesian stallion, you’re starting with a foal gestating in a large-frame, high-draft-capacity uterus—with a slow-growth, heavy-bodied metabolic blueprint. But flip it—Friesian mare to Shire stallion—and you’ve got a foal developing in a tighter, baroque-bodied space, with a different hormonal and energy signaling environment.
These intrauterine differences shape the phenotype well before the foal takes its first breath.
Within the Friesian breed itself, there’s notable diversity that impacts crossbreeding outcomes. Traditionally, Baroque (or Classic) Friesians display the breed’s hallmark compact, muscular, heavily feathered, and thick-necked body type, which strongly influences offspring phenotype when used as a parent.
On the other hand, Modern Sport Friesians—bred more recently for dressage and sporthorse disciplines—are often taller, leggier, leaner, and less heavily feathered. This shift toward a more modern, athletic build means their foals tend to inherit a lighter frame and more sport-type conformation compared to those from Baroque Friesians.
Thus, crossing a Baroque Friesian (whether as sire or dam) with a Gypsy, Shire, or Clydesdale typically produces foals with a dense, powerful build, pronounced feathering, and classic draft-type features. But using a Modern Sport Friesianin the same crosses often yields offspring that are taller, less bulky, with finer limbs and reduced feather—sometimes leaning toward a sporthorse look rather than the traditional heavy appearance.
This distinction is crucial for breeders to consider when planning crosses, as the type of Friesian parent can dramatically alter the foal’s appearance and athletic potential—even with the same breed combinations.
Another layer of complexity lies in something called genomic imprinting, where the origin of a gene (sire or dam)determines whether or not it gets expressed.
Some genes—particularly those involved in growth regulation, like IGF2—are only “turned on” if inherited from one parent, typically the sire. These imprinted genes can affect:
*How quickly the foal grows
*Final body size
*Muscle and bone development
*Fetal programming and nutrient absorption
So if your stallion comes from a heavy, large-boned breed like a Shire or Clydesdale, and those growth-promoting genes are paternally expressed, they’re more likely to show in the foal—but only if the foal inherits them from him.
When combined with the maternal effects above, imprinting contributes to the consistent observation that Shire stallion × Gypsy mare tends to produce more drafty, cob-type foals, while Gypsy stallion × Shire mare results in taller, lighter, often more refined crosses.
Traits like feathering, height, body type, and even temperament aren’t controlled by a single gene. They’re polygenic, meaning they arise from the interaction of many genes—each contributing small amounts to the overall outcome.
Because of that, polygenic traits can behave unpredictably, and are also strongly influenced by maternal environment and gene dominance. A Shire may pass on genes for massive size, but if those genes land in the uterine environment of a smaller mare (like a Gypsy), fetal growth will likely be restricted, resulting in a more compact horse.
Likewise, two Gypsy-sired crosses may both carry feathering genes, but if one is out of a Shire and the other out of a Friesian, the amount and quality of feathering, bone, and shape can differ significantly.
This also explains why some foals from the same cross look extremely “typey,” while others seem like outliers—it’s the result of polygenic inheritance interacting with fetal development dynamics.
The big takeaway? The order of the cross truly matters—in almost every meaningful way. It affects:
*Skeletal development and limb proportion
*Expression of polygenic traits like feather and height
*Growth rates and body mass through imprinting
*Metabolism and endurance via mtDNA
*Even the overall health, strength, and refinement of the foal
This is why a Friesian stallion to a Shire mare can produce a horse that matures slowly, grows tall, and shows light feather—while a Shire stallion to a Friesian mare may give you a denser, heavier foal with more draft characteristics. It's the same genetic material, but the roles of dam and sire shift how those genes are interpreted and expressed.
And just like with mules and hinnies, same ingredients, different outcome—simply based on who carries the foal.
So the next time you’re planning a cross, evaluating a foal, or even just admiring a friend's new baby horse, think deeper than just the breed percentages. Ask yourself:
Which parent was the dam?
What kind of environment shaped that foal in the womb?
What kind of traits are maternally inherited or suppressed?
Understanding these patterns doesn't just make us better breeders—it helps us appreciate the complex, beautiful biology behind every horse.
Zazu F1 North American Destrier Stallion
Owned and bred by NADHR member Wendy Morris.
Friesian sire Gypsy Dam.
