Inbreeding, Outcrossing and Line Breeding
Inbreeding
Inbreeding refers to mating sires and dams that share common ancestors within five generations or less. This is done to increase the chances that the foal will inherit traits from the duplicated ancestor. More than one ancestor can be duplicated in the same pedigree.
Inbreeding can have advantages...
...but there are also disadvantages:
Some breeders believe that inbreeding to a female with superior qualities as a racer or producer is especially effective. Such inbreeding is often referred to as the "Rasmussen factor" after Leon Rasmussen, who advocated this approach.
Inbreeding is most effective when the "to and through" rule is followed: Inbreed to good ancestors through their good descendants.
Inbreeding can have advantages...
- It can concentrate the genes of a superior ancestor.
- It can fix a desired type relatively quickly.
- It may result in animals that are more likely to transmit their own traits regularly when they are used for breeding.
- It may be easier to predict which crosses will work well with an inbred animal when it is used for breeding.
...but there are also disadvantages:
- Inbreeding to an ancestor increases the risk of inheriting any undesirable characteristics it carried.
- Inbreeding raises the chance of a foal inheriting two copies of recessive genes for an undesirable trait or a genetic disorder.
- Intense inbreeding may result in loss of vigor in the resulting stock.
Some breeders believe that inbreeding to a female with superior qualities as a racer or producer is especially effective. Such inbreeding is often referred to as the "Rasmussen factor" after Leon Rasmussen, who advocated this approach.
Inbreeding is most effective when the "to and through" rule is followed: Inbreed to good ancestors through their good descendants.
Outcrossing
Thoroughbreds described as "outcrossed" do not have any ancestors duplicated between the sire's and dam's sides of their pedigrees within five generations. If an ancestor is duplicated on the sire's side of the pedigree but not on the dam's side -- or vice versa -- the foal is still considered to be outcrossed.
Breeders make use of outcrossing to
The primary drawback of outcrossing is that outcrossed individuals are less likely to transmit their own type because of their more varied genetic backgrounds. It can also be harder to predict what bloodlines will work well as a cross.
Some breeders alternate inbreeding and outcrossing in subsequent generations to take advantage of the strengths of both methods. One "best of both worlds" approach involves taking an inbred parent and breeding it to a mate free of the duplicated ancestor but sharing a different superior ancestor with the inbred animal. The resulting foal will be outcrossed with regard to the first ancestor but will have a new inbreeding to the second ancestor.
Breeders make use of outcrossing to
- Increase the vigor of their stock, especially when one or both of the parents to be used are strongly inbred,
- Draw on a wider genetic pool, and
- Reduce the probability of a foal's inheriting two copies of an undesirable recessive gene.
The primary drawback of outcrossing is that outcrossed individuals are less likely to transmit their own type because of their more varied genetic backgrounds. It can also be harder to predict what bloodlines will work well as a cross.
Some breeders alternate inbreeding and outcrossing in subsequent generations to take advantage of the strengths of both methods. One "best of both worlds" approach involves taking an inbred parent and breeding it to a mate free of the duplicated ancestor but sharing a different superior ancestor with the inbred animal. The resulting foal will be outcrossed with regard to the first ancestor but will have a new inbreeding to the second ancestor.
Line breeding
Line breeding attempts to gain the benefits of inbreeding while reducing the risks. It does so by accumulating multiple crosses of one or more superior ancestors, typically at the fifth generation and beyond. The hope is that through selective breeding and weeding out of undesirable traits over several generations, the foal can inherit a wider selection of the ancestor's beneficial genes through several of the ancestor's good sons and daughters. Line breeding to multiple good ancestors in a pedigree tends to indicate compatibility between the sire's and dam's ancestry.
The great stallion Northern Dancer is an excellent example of a line bred pedigree. While on the surface he was only mildly inbred with a 4x5 cross to Gainsborough (one cross at the fourth generation and one at the fifth), he had 23 crosses to the great English racer and sire St. Simon within the sixth through eighth generations of his pedigree. He also had five crosses (6x6x6x8x7) of the important mare Canterbury Pilgrim; a 4x6 cross of another important mare, Selene; a 4x6 cross to the top sire Polymelus; and a 5x6 cross to Derby Stakes winner Spearmint.
The great stallion Northern Dancer is an excellent example of a line bred pedigree. While on the surface he was only mildly inbred with a 4x5 cross to Gainsborough (one cross at the fourth generation and one at the fifth), he had 23 crosses to the great English racer and sire St. Simon within the sixth through eighth generations of his pedigree. He also had five crosses (6x6x6x8x7) of the important mare Canterbury Pilgrim; a 4x6 cross of another important mare, Selene; a 4x6 cross to the top sire Polymelus; and a 5x6 cross to Derby Stakes winner Spearmint.