Inbreeding and outcrossing
This article is only meant to give a brief overview of inbreeding: what it is, how it is done and pros and cons. Outcrosses seen in relation to inbreeding, is also briefly discussed. For general linebreeding strategies, please see separate article.
If you are unsure of the definition of inbreeding, or if you are unsure if your use and understanding of inbreeding matches our definition, please consult the Definitions article first!
Inbreeding attempts to concentrate the genes of a specific ancestor (or ancestors) by mating two individuals who both have that ancestor at least once in their pedigree and in a recent generation. Used as an example in the Definitions article, Roxane Griff is inbred 3x3 on Chambon P. Both her sire and dam have Chambon P as their siresire (paternal grandsire)
When you inbreed like this you double up on certain genes and genetically you concentrate the traits, both good and bad. If you inbreed on a spectacular individual the goal is to double up on the good genes and produce an individual that represents all the good characters of Chambon P. However, it is important to repeat "you concentrate the traits, both good and bad." The bad is as important as the good. While the hope and focus of the breeder is to get all the characteristics that made Chambon P a good racehorse and a great stallion, and to get all the good characteristics that his good foals exhibit, the bad characteristics may end up dominating genetically. If the genes for a conformation fault are present, but hidden or resessive so they do not show up in a son or daughter, inbreeding has a higher probability of bringing them out since you can double up on that gene (once through the sire and once through the dam). So a problem that does not show up in a son or daughter alone can show up when you inbreed on a horse. If the genes for other undesirable traits are present, they are more likely to appear as well. So a partial conformation fault may be "intensified" through inbreeding and negate any positive effects otherwise obtained.
Generally speaking, the term "inbreeding depression" describes the reduced fitness in a given population as a result of inbreeding and is the expression generally used to describe the downsides to inbreeding. In addition to fixing the desired traits in the breed, inbreeding also fixes (or increases in frequency) deleterious recessive traits that are genetically linked to the desired traits. ("Genetically linked" simply means that the two genes are located on the same chromosome, close to one another). Many of these deleterious recessive alleles may have only small effects (even when homozygous), but as more of them become fixed or increase in frequency in the population, the fitness of the inbred animals almost always suffers. Thus, fitness usually declines upon inbreeding. If the finer details or genetics are difficult, just accept the conclusions that
1) that inbreeding reduces genetic variation
2) inbreeding can (but does not have to) strengthen or intensify both positive and negative genetic traits inherited from common ancestors
One should not be blind to the word "inbreeding" while ignoring "linebreeding." A horse that is "only" linebred can still have a high degree of inbreeding, especially if several horses in its pedigree is linebred on the same horse.
WHY INBREED AT ALL
Given the risk of negative effects, why inbreed at all?
The establishment of the standardbred began with a lot of inbreeding. Hambletonian 10 possessed all the right genes for his foals to be good trotters. It was not a genetic trait unique to Hambletonian but he exhibited it to a very high degree. When breeders inbred to Hambletonian they were hoping to double up on his trotting genes (and certainly the conformation that came with the ability to trot). Inbreeding can significantly increase uniformity in the breed which is important when establishing a new breed. Some horses inbred on Hambletonian did not get a desired genetical concentration of these genes and did not stand out as trotters (or pacers), but those who did get the desired genes often became very good racehorses and several of these are part of the foundation of the breed. One such example is world record setter Nancy Hanks who is 2x3 inbred on Hambletonian 10.
Historically, inbreeding is in fact how many of the desirable and distinctive characteristics of the standardbred came to be established.
This positive effect is not limited to old times. For a few great stallions the term "breed changer" is applied to stallions who are extremely dominant and successful, such as Peter the Great, Meadow Skipper and Speedy Crown. Inbreeding on these horses tend to produce some very good individuals. Speedy Crown at stud exhibited many of the desired genetic traits and concentrating his genes by inbreeding on him has contributed to the progress and development of the strandardbred.
According to the blog "Have a very inbred christmas", Charles Darwin is said to have noted that "the negative effects of inbreeding do not become evident immediately, but the positive effects do." This can explain why some very inbred individuals do very well on the track but at the same time it is also saying that you have only delayed, and not avoided, the negative effects.
ACCEPTABLE LEVELS ON INBREEDING
There is no defined acceptable level of inbreeding and although most harness racing associations generally advise against too much inbreeding, nobody has set any restrictions. Generally in livestock breeding it is not recommended that the level of inbreeding in a population go above 6-7 %. Inbreeding depression is also said to likely be more apparent when inbreeding levels is above 10%.
Several studies have noted a marked reduction in both starting percentage and average earnings when the inbreeding coefficient becomes too high or the inbreeding pattern is very tight (for example a 2x2 inbreeding). However, there are always some very inbred and extraordinary racehorses that stand out as exceptions. Some people tend to see these exceptions as validating tight inbreeding. This is statistically not valid and it must be rememeber that without these exceptions the average numbers of the rest of the inbred horses are even weaker. There is some disagreement to the "best levels of inbreeding" but there is no question that at some level of inbreeding the effects become negative.
Inbreeding in an individual must be viewed relative to the population. If the whole population is quite inbred then a very inbred individual will not differ much. So if the population as a whole has a 15 % inbreeding coefficient then a horse with a 20 % inbreeding coefficent is not that different, but if the population on average has a 6 % inbreeding coefficient then individual horses with a 20 % inbreeding coefficient differs a lot more and may deviate much more from the performance averages.
As was quoted above, "the negative effects of inbreeding do not become evident immediately, but the positive effects do", so even though it has worked before that is no reason that it will work again. Inbreeding of today is often a matter of inbreeding on already inbred individuals which is different than inbreeding on individuals that are not inbred. To inbred on Speedy Crown in one thing, but if you breed two individuals both inbred on Speedy Crown you run a much higher risk of the negative effects of inbreeding. There are still some great horses bred on this pattern (both parents of Ken Warkentin are inbred on Speedy Crown, both parents of Viking Kronos are inbred on Star's Pride and both parents of Cam Fella are inbred on Hal Dale) but the average earnings of this group is lower.
There is a view that says that the inbreeding pedigree pattern matters more than the inbreeding coefficent (i.e. so a horse inbred on a 2x3 pattern with 15 % inbreeding coefficient is "worse" than a horse linebred 4x4 but has a 17 % inbreeding coefficient). Expect this to be the subject of some articles found in the Article section.
There is also a view that breeding success has more to do with the quality of the sire and dam rather than the potency of inbreeding: so inbreeding on Speedy Crown through poor sire and dam in itself is "useless." There are no significant analyses that have specifically explored this view more in depth at this time.
OUTCROSSING INBRED INDIVIDUALS
Some breeders have used inbreeding as the first of a two-step breeding strategy (or they purchase inbred broodmares). The theory of "outcross inbred individuals" is based on the concept of "heterosis" which, a bit simplified, is the reversal of inbreeding depression. It is very important to point out that not all outcrosses result in heterosis. For example, when an outcrossed foal inherits genetic traits from its parents that are not fully compatible, fitness can be reduced. This is, in fact, a type of outbreeding depression and explains why some Franco-American outcrosses are great trotters while others have very little trotting ability at all.
Although studies has shown that severely inbred individuals tend to underperform as racehorses many function exceptionally well at stud and many of the best racehorses have one or two inbred parents. Although one cannot say that heterosis made those horse great it surely was one of many factors. Several successful breeders actively look for heavily inbred mares to practice the "outcross inbred individuals" strategy.
Muscle Hill has two inbred parents, Muscles Yankee is inbred on Speedy Crown and Yankee Blondie is inbred on Star's Pride.
The Italian stallion champion Sharif di Iesolo has a dam that is 2x2 inbred to Kairos
Commander Crowe has a dam that is heavily inbred: Both of Somack's parents, Mack Lobell and Somolli Ribb are inbred on Speedster (3x3 and 2x4 respectively). Speedster first appears in the fifth generation in Juliano Star's pedigree.
It is important, however, to emphasize that outcrossing on inbred inviduals is a strategy but no guarantee.