BY ED BALCEWICH, DESIDERIUS FARMS, ST. ANDREWS, MB, CANADA
GRAPHICS: ED BALCEWICH

I had a conversation last season with a breeder – knowledgeable, well-read, the kind of person who has clearly put real years into this. He asked me a fair question: what was I actually trying to breed into our horses? More uphill? Lighter? Faster? Better brain? More jump

It’s the question most of us are handed early, because it’s the language the sport has always used. You look at the horse in front of you, you decide what you’d change, and you go looking for a stallion who seems to have more of it.
It treats breeding like ordering options on a new car – pick the trim, pick the upgrades, and the dealer builds it to spec. Breeding does not work that way. There is no gene for “more uphill.” There is no allele you can request for “better brain.” These are polygenic traits, meaning dozens of genes, scattered across different chromosomes, each contributing a small effect, reshuffled fresh at every single breeding. You cannot select the option. You can only load the deck.
I told him the truth as I understand it – that we weren’t chasing any single trait. We were trying to bring forward as much recombination of the proven foundation blood as we could, generation over generation, and let the traits that mattered fall out of that architecture rather than chase them one at a time.
He didn't like the answer. Something in it – maybe the refusal to name a single trait – seemed to frustrate him, and he stopped listening not long after. The conversation ended there, and I don’t think either of us walked away feeling we’d been heard.
I’ve thought about that exchange more than once since. He wasn’t wrong to ask the question the way he did – it’s the question the sport trained him to ask. I wasn’t wrong to answer the way I did, either. We were just standing on opposite sides of a gap that doesn't get talked about enough: the difference between breeding for what you can see in a horse, and breeding for what’s carried in the deck underneath it. That gap is worth closing. This is my attempt at it.

Two decks, not one

Think of it as two decks of cards, not one. The sire brings his deck. The dam brings hers. Every card in both decks is an allele – one specific version of a gene, inherited whole from a parent, not built to order. A horse doesn’t have “a gene for scope.” It has two copies of every gene, one from each parent, and which version – which allele – it received shapes what that gene actually does. Each horse carries roughly 40,000 of these cards in total – 20,000 from the dam, 20,000 from the sire. Every card comes in a pair
Here’s the part of the shuffle that matters most, and it happens before either parent ever reaches the breeding shed. At every single position in the deck (a locus) – every one of those roughly 20,000 genes – the stallion isn’t holding one card. He’s holding two, stacked together: one handed down from his own sire, one from his own dam. Same position, two possible cards, every time. Mom is holding the same thing at every position – two cards of her own.
When a sperm cell gets built, it doesn’t grab a whole prebuilt hand from one side or the other. It walks the entire deck, position by position, and at each one takes a single card off the stack – sire’s-side card or dam’s-side card. Nearby positions tend to get picked together, riding the same stretch of chromosome, until a crossover splits the run and starts picking from the other side of the stack instead. That's the shuffle happening in real time, thousands of times over, across every chromosome – the same thing happens independently when the mare’s egg is built.
The result is one hand, freshly assembled, that belongs to that sperm cell or that egg alone. Some of its cards trace back through the sire’s own sire, some through the sire's own dam; and on the dam’s side, the same split happens independently, some cards tracing back through the dam’s own sire, some through the dam’s own dam. The mix is never sorted cleanly into two neat piles; it’s interleaved, card by card, in a pattern no other reproductive cell will repeat exactly.
That's why two full siblings can carry noticeably different weight of the same ancestor, even out of the same two parents. Both parents are holding the same two decks each time. But every sperm and every egg walks those decks independently, taking a different card at enough positions along the way that the final hand looks like its own thing. One sibling might come up holding more of a superstar sire's cards. The other might come up light on that same superstar, and heavier somewhere else entirely.
Do that across roughly 20,000 positions, and the foal ends up holding a hand of 40,000 cards of its own – 20,000 from the sire, 20,000 from the dam – a brand-new combination that has never existed before and will never exist again in exactly that arrangement...

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