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There are 4 forms of breeding, namely-


Outbreeding is the mating of two animals from unrelated animals.  It is different to out-crossing; here neither of the two animals are products of line-breeding programs. The outbreeding breeder intends to remove certain traits by using "new blood", but not wanting any potential genetic bottle-necking that may exist in line-bred individuals.


Outcrossing is the crossing of two bloodlines if you will, where both animals come from line-bred stock. Out-crossing introduces unrelated genetic material into a breeding line. This increases genetic diversity and reducing the probability of all individuals being subject to genetic deformities and disease. It is used in line breeding to restore vigour or size and fertility to a breeding line. However, it can actually increase the number of individuals who carry a disease recessively.

The outcrossing breeder intends inject certain desirable traits found in the blood-line he is outcrossing to. With dominant traits, one can still see the expression of the traits and can remove those traits whether one outcrosses, line breeds or inbreeds. With recessive genes, outcrossing allows for the recessive traits to migrate across a population. It may actually increase the number of individuals carrying a disease. The outcrossing breeder then may have individuals that have many detrimental genes that are expressed by placing their animals against a similarly outcrossed individual. There is now a gamut of detrimental genes within each individual in many breeds.


Inbreeding; Breeders inbreed within their genetic pool, attempting to maintain desirable traits. When undesirable traits begin to appear, matings are selected to determine if a trait is recessive or dominant. Removal is accomplished by breeding two individuals of known genetic status, usually they are related.

Inbreeding depression is reduced fitness in a given population as a result of breeding of related individuals. Breeding between closely related individuals, called inbreeding, results in more recessive harmful traits manifesting themselves. The more closely related the breeding pair is, the more homozygous detrimental genes the offspring may have, resulting in very unfit individuals. Another mechanism responsible is over-dominance of heterozygous alleles leading to a reduction in the fitness of a population with many homozygous genotypes, even if they are not detrimental. Currently it is not known which of the two mechanisms is more important. In general, populations with more genetic variation do not suffer from inbreeding depression. Inbreeding depression is often the result of a population bottleneck

Introducing new genes from a different population can reverse inbreeding depression. Different populations have different deleterious traits, and therefore will not result in homozygosity in most loci in the offspring. This is known as out-breeding enhancement, practiced by conservation managers and zoo captive breeders to prevent homozygosity. However, intermixing two different populations may give rise to unfit polygenic traits in out-breeding depression.


Outbreeding depression refers to cases when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population. This can occur in two ways. First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population.

A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different populations. Due to nonadditive gene action, genes may have rather different effects in different genetic backgrounds.

In other words, individuals from Population A will tend to have genes selected for the quality of combining well with gene combinations common in Population A. However, these will not have been selected for the quality of crossing well with genes common in Population B. Therefore outbreeding can undermine vitality by reducing positive epistasis and/or increasing negative epistasis.

It is critical to understand that reduced inbreeding depression in first generation hybrids can, in some circumstances, be strong enough to more than make up for outbreeding depression. This is why farmers keep purebred strains for the purpose of outcrossing (while not usually breeding the hybrids).

As a general rule of thumb, hybrid vigour (or reduced inbreeding depression) is strongest in first generation hybrids and gets weaker over time. In contrast, outbreeding depression can be relatively weak in the first generation, and will increase through the further generations as co-adapted gene complexes are broken apart without the forging of new co-adapted gene complexes to take their place.

It is important to keep in mind that these two mechanisms of outbreeding depression can be operating at the same time. Determining which mechanism is more important in a particular population is very difficult.


Linebreeding is a form of inbreeding practiced by some animal breeders to "fix" desirable traits in a breed of animal, without as high a risk of producing undesirable traits that may occur with close inbreeding.

A typical example of linebreeding would be what in human parlance would be considered a mating of first cousins or more distantly related individuals who share a common ancestor.

While linebreeding is less likely to cause problems in the first generation than does inbreeding, over time, linebreeding can reduce the genetic diversity of a population and cause problems related to a too-small gene pool that may include an increased prevalence of genetic disorders and inbreeding depression.

In the creation of the Old English Bulldogge - Mr Levitt uses an adapted form of the Ohio University Linebreeding Program. His method to form a bloodline is to start with a dog and a bitch which are entirely unrelated, then mate the best bitch progeny to again an entirely unrelated dog. From this point onward he linebreeds the next generation Best Bitch to an Uncle from the previous generation.

This does indeed have it's merits in forming your own bloodline, and is better than Father-daughter, or brother-sister matings. However, as stated above, prolonged linebreeding of any sort causes genetic bottle-necking, and soon genetic disorders will start being displayed.

This can be readily be seen in the "Test Bloodline" I created on the database, by looking at the incremental Coefficient of Inbreeding with each successive generation. Go to the Database and see the Inbreeding Pedigree for these Test Children. Or View the Coefficient of Inbreeding here




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