What Is Linebreeding?

At its simplest, linebreeding means deliberately having the same proven, high-quality ancestor appear more than once in a foal's pedigree, usually somewhere in the 3rd to 6th generations, and usually on both the sire's side and the dam's side.

Why breeders bother

The thoroughbred was created centuries ago by crossing native British mares with imported Arabian, Barb and Turkish stallions. Because of that mixed ancestry, racehorses rarely "breed true". Mating two horses is a bit like shaking a kaleidoscope: you get a different mix of traits every time. It's why a champion so often has a very ordinary brother or sister.

Linebreeding is an attempt to tilt those odds. By bringing a great ancestor (think Northern Dancer, Nearco or Danehill) into the pedigree more than once, breeders are trying to stack the deck with more aces, and increase the chance the foal inherits the qualities that made that ancestor great.

How to read "4x5"

You'll often see linebreeding written as two numbers, like 4x5. It's simple:

The first number is the generation the ancestor appears in on the sire's side.
The second number is the generation on the dam's side.

Count the sire and dam as generation 1, the grandparents as generation 2, and so on. So "Try My Best 4x5" means Try My Best appears four generations back through the sire, and five generations back through the dam.

Linebreeding vs inbreeding: what's the difference?

Strictly speaking, linebreeding is a form of inbreeding. The difference is how close up the duplication is.

Close inbreeding means very close relatives, like father to daughter or brother to sister, with the duplicated horse in the 1st or 2nd generation. It concentrates traits quickly, but it carries a much higher risk of what scientists call "inbreeding depression": problems such as lower fertility, weaker constitution and reduced performance.
Linebreeding keeps the duplicated ancestor further back, usually the 3rd to 6th generations. The risks are much lower, though not zero, and the aim is to concentrate a great ancestor's influence without the problems of close inbreeding.

Who used it?

Linebreeding isn't a new idea. Some of the most famous names in breeding history built their reputations on concentrating great ancestors.

Federico Tesio, "the Wizard of Dormello", is widely regarded as one of the greatest breeders in history. The Italian bred the unbeaten champions Nearco and Ribot, and was famous for studying pedigrees obsessively and concentrating the blood of superior ancestors.
Marcel Boussac, the French textile tycoon, dominated French racing in the mid-1900s. He went further than most and was known for unusually close inbreeding to his own foundation stock, believing that doubling up on his best bloodlines would "fix" the type he wanted.
Leon Rasmussen and Rommy Faversham popularised the Rasmussen Factor (RF): inbreeding to an outstanding mare (famous examples include La Troienne and Almahmoud) through two different sons or daughters, within five generations.
Clive Harper and Ken McLean, two respected pedigree writers, studied large numbers of stakes winners and argued that elite horses tend to carry a concentration of linebreeding in their 4th, 5th and 6th generations.

Why do breeders use it?

Breeders who linebreed are usually trying to do four things.

1. Reduce the randomness. Concentrating the same ancestor narrows the range of genetic outcomes, with the aim of producing more consistent results and fewer wild surprises.

2. Bring back a great ancestor's influence. On average, each ancestor's share of a foal's genes halves with every generation: 50% for a parent, 25% for a grandparent, and only around 6% by the 4th generation and 3% by the 5th. Linebreeding brings that ancestor back in a second time, so the foal has two chances, not one, to inherit their genes. Tesio wrote about this idea of "replenishing" a great ancestor's influence before it faded away.

3. Create prepotency. A prepotent horse is one that stamps its offspring, passing on its qualities reliably. Here's the genetics behind it: when a horse carries two identical copies of a gene, it passes that same version to every single foal. (A C/C "Dragster" passes on a speed copy every time, for example.) Linebreeding increases the chance of carrying identical copies, which can make a horse more prepotent. The catch is that this works for weaknesses as well as strengths, which is why who you linebreed to matters so much.

4. Achieve sex balance. When the duplicated ancestor comes through both a son and a daughter, rather than through two sons, many pedigree students (me included) believe it gives the pedigree the chance to reunite both the male and the female genetic strains of that bloodline. This is a theory rather than proven science, but it's one of the patterns I look for most.

A real example

Ka Ying Rising, the champion Hong Kong sprinter, is linebred 4x5 to Try My Best, and it's sex-balanced: one line comes through a son (Last Tycoon) and the other through a daughter (Daffodil Fields). Read the full Ka Ying Rising story.

The bottom line

Well established: Every ancestor's influence halves, on average, with each generation, and duplicating an ancestor increases the chance a foal inherits their genes.
Theory and experience: Linebreeding to the right ancestors, at the right distance and with sex balance, is a pattern many of the great breeders and pedigree students swear by. It's not a guarantee, and it's not a formula. It's a way of stacking the odds.

Michael MarosaThe Pedigree Nerdmichael@digitalbloodstock.com

Sources

Rasmussen L & Faversham R (1999). Inbreeding to Superior Females.
Tesio F. Breeding the Racehorse.