Line breeding, close breeding, inbreeding in sport horses

Wow, I just read a great article in the Fall 2002 issue of Holsteiner magazine: Linebreeding, Inbreeding, Close Breeding A primer covering the Tesio Method, the Rasmussen Factor, and other breeding theories. I anticipated a dry but instructive read, but it was actually fascinating. I'm going to share a few of the basics from the article, but I recommend you read it yourself if you're interested in the subject.

"Isn't that like marrying your cousin?"
Several years ago, when a friend first described line breeding to me, it seemed kind of creepy. If you haven't heard this term, the average person would probably think of it as inbreeding--when a sire or dam appears more than once within a pedigree. The goal of line breeding is to concentrate bloodlines to bring out desirable traits in the resulting offspring.

It's all about what genes influence the offspring. Geneticists say that a parent must carry 10% of a desired bloodline (e.g., Bold Ruler) in order for that blood to have any influence on the offspring. A son or daughter will have 50% of a sire's blood, a grandson or granddaughter will have 25%, and a great grandson or great granddaughter will have 12.5%. Blood from the great-great grandsire would be less than 10% (no influence). Line breeding or inbreeding will increase the genetic influence of a bloodline. Here are the definitions of line breeding and related terms.

Line breeding
A horse is considered "line bred" or the product of line breeding when he or she has a common ancestor outside the fourth generation. According to the article, linebreeding "is duplicating key forbears in the fourth to sixth generation." An example of line breeding is the thoroughbred Fairway. If you look at his pedigree you'll see where horses are repeated. It isn't just the presence of the horses that matters, it's where they appear (sire or dam line, etc). Theories of line breeding can be very, very involved.

In the article Pedigree generation position, the author Kathleen Kirshan states that "the focal linebreeding should be in what many call the engine room, which is the 4th through 6th generation positions." If you do choose to read more, it helps to know there is a notation used to describe the pedigree in terms of line breeding: 4X4 indicates the same horse is presesnt in the fourth generation two times; 3X6 means the horse is present in the third and the sixth generation; and so on.

Inbreeding
Inbreeding in the vernacular is a general term for what happens when you marry a relative. In horse breeding, it means duplicating an ancestor in the first four generations of an animal's pedigree. An example of inbreeding is found in the warmblood stallion Matcho.

Close breeding
Mating between brothers and sisters, or between parents and their off-spring, is referred to as close breeding. It is seldom done in real life breeding, although the spectacular jumper Rex Z is the product of a full brother/full sister mating. Also a few years ago I read in the USDF Connection that Hilda Gurney bred a daughter to hers sire. I believe this horse was a year end award winner -- but it's still creepy in my book. Close breeding is often used in genetic research to expose genetic weaknesses quickly.

Risks of inbreeding/close breeding
The same mechanism that strengthens the desirable traits also exposes weaknesses in the genetics of a bloodline. Too much inbreeding in labrador retrievers and german shepherds led to the problem of hip dysplasia. Inbreeding can cause runty, unhealthy, infertile, bad-tempered animals. Most breeders try hard to balance line breeding with outcrossing.

Outcrossing
Secretariat was outcrossed, meaning he was the product of two unrelated strains. This results in what they call "hybrid vigor," and in the horse world it can produce super-horses like Secretariat. But while outcrossing is more likely to produce healthier more fertile, and stronger individual, it's less likely to produce a good sire. Because their genes are heterozygous it is harder to predict what their offspring will be. In contrast, line bred horses will have homozygous genes. There is less variaility in their genetic makeup so their offspring will more predictably inherit the traits of their sire or dam. A stallion that produces the same traits consistently is said to be prepotent. A prepotent stallion consistently passes on desirable characteristics to offspring. The trait can be speed, endurance, jumping talent, dressage gaits, or beauty/conformation, for example.


Takeaway message
While homozygous genes (from line breeding) make it easier to predict what the offspring will be like, heterozygous genes (from outcrossing) make for a healthier animal. Most breeders will agree that a pedigree should be balanced with respect to outcrossing and line breeding. Avoid extremes in either direction!

RESOURCES
Reading pedigrees from The Horse

Chief Bearheart: The power of line breeding

Sport-horse-breeder.com

Is sire line outcrossing possible any more? from the Blood Horse