Inbreeding and Outcrossing—What Does It All Mean?
In the world of horse breeding—particularly Thoroughbreds—you’ll often hear the terms inbreeding and outcrossing. They sound like complicated scientific processes, but they boil down to how breeders try to stack the odds in their favour when creating a future champion.
To understand these terms, let’s first set the scene:
All modern Thoroughbreds come from a very limited genetic pool—just four foundation stallions and around 50 to 80 foundation mares, depending on which historical point you’re referencing (such as the Jersey Act). That means every Thoroughbred today is, to some extent, inbred. So when we talk about an “outcross,” we mean a horse that shows no close duplication of ancestors for a certain number of generations—usually six or seven.
Inbreeding: Stacking the Deck with Known Winners
In simple terms, inbreeding is when a breeder mates horses that share a common ancestor fairly close up in the pedigree—for example, a stallion that appears in both the sire and the dam’s side, say in the 2nd and 3rd generations (e.g. Enable, inbred 3x2 to Sadler’s Wells).
The idea here is logical: if that ancestor was exceptional, then doubling up on their genetic material may increase the chance of recreating their qualities in the foal. This is sometimes referred to as “fixing” certain traits in the bloodline.
However, genetics isn’t that predictable.
Thoroughbreds are hybrids, subject to Mendel’s laws of inheritance. Each time a mating occurs, the genes from both sire and dam are shuffled and grouped in what’s called the law of independent assortment. This means that every new individual is truly unique—no perfect carbon copy of a past ancestor can be reproduced just by repeating a name in the pedigree.
So while duplicating a name like Danzig in a pedigree might look promising on paper, the actual genes inherited from him will vary every time. The same name in a pedigree does not guarantee the same genetic result.
That said, judicious inbreeding, when done with knowledge and care, can help concentrate desired traits, especially when backed by a sound understanding of how the genes of those ancestors actually influence performance or soundness.
Outcrossing: Mixing It Up for Hybrid Vigour
Outcrossing is a simpler and more common-sense approach.
Instead of repeating the same names in a pedigree, breeders aim to mate horses that have no recent common ancestors. This practice is meant to bring about what’s called hybrid vigour—a biological phenomenon where genetic diversity leads to increased vitality, fertility, or performance.
It’s also used to try new combinations, often referred to as “nicks”—when two unrelated bloodlines, for whatever reason, seem to consistently produce superior offspring when crossed.
Outcrossing doesn’t guarantee a champion either, but it avoids the risks of over-concentrating weak traits and gives breeders room to experiment with fresh genetic combinations.
A Final Thought
Inbreeding and outcrossing are both tools. One isn’t better than the other—they simply serve different purposes. Inbreeding seeks to consolidate proven excellence. Outcrossing aims to introduce new potential.
Both are subject to the randomness of genetics and the unpredictable nature of biological inheritance. But with a careful eye, a knowledgeable breeder can use either method to tilt the odds ever so slightly in their favour.