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· OCD Bullyologist
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By Scot E. Dowd Ph.D.

Willis (1989) defines INBREEDING as the mating of animals "more closely related to one another than the average relationship within the breed." From the introductory table we can see that these involve brother/sister or parent/child breeding.
With all the negative connotations associated with inbreeding why discuss it as a viable breeders tool?
It is true inbreeding can create horrific genetic mistakes or it can be the best and fastest way to improve and standardize a line (set traits). IN the first sense it is a useful genetic tool to reveal hidden recessive genetic problems in your line by doubling up on them and making them express themselves (as it were LOL). Note this is a tool used to reveal hidden defects and these defectives should be culled from the breeding pool. In the second sense when your line is heterotrophic for beneficial traits you can do inbreeding to make them homozygous (set the trait). Note it must be a tremendously important trait or preferably numerous traits you are trying to set within your line to consider inbreeding as the best method to set the trait. So here inbreedings are done to verify the superiority of a particular strain or to check for recessive problems. Any weaknesses, faults, deformities, etc. are likely to show up. If a truly superior line or strain has been developed, containing only desirable qualities, these desirable qualities will be seen in the resulting offspring.

There is a great example of the result of professionally developed and planned inbreeding that against all odds has gone right (don't EVEN CONSIDER OR HAVE A FANTASY that you can reproduce this with the APBT!) Anyways, I copied the following paragraph from another site.
Guiding eyes For The Blind?, Inc., located in San Raphael, Ca., has developed 3 strains of German Shepherd Dogs to guide the blind. One of these strains, the "Frankie line" is being inbred to produce offspring containing as much of Frankie's genetic material as possible, some having as high as 96% of their genetic material from Frankie. And these dogs are happy, healthy, well-adjusted and successfully guiding their blind owners through city traffic, etc. (Another strain is geared to linebreeding to produce puppies that have as close to exactly 50% of Frankie's genes as possible-linebreeding. The third strain is based on another dog Orthos, and they are trying to increase the percentage of his genes in the puppies to the highest possible extent, while still maintaining something genetically from Frankie.)

The totality of qualities belonging to that individual AND the knowledge of the breeder will determine whether this is a horrible thing or a wonderful thing. As I mentioned previously to a geneticist linebreeding and inbreeding only differ in degree measured by the "inbreeding coefficient" which puts a number to the degree of inbreeding an animal shows relative to a random breeding population.

As animals are mated to their relatives, however distant, simple mathematics will show that the likelihood of any one gene becoming homozygous will increase. As homozygosity increases, variation among offspring decreases. The dog breeder takes advantage of this in inbreeding to produce a breed which "breeds true" and conforms to a "Breed Standard" and within the breed to produce offspring that are like peas in a pod (all have Breed Type?). breeders look for a "prepotent" sire or bitch ( ie one that always throws pups very similar to itself). These animals come from a "good line bred pedigree" - that is one that is inbred so that the animal is homozygous for as many as possible of the characteristics that the breeder regards as desirable.

Unfortunately this search for perfection and uniformity comes at a cost. Undesirable genes also become increasingly likely to be homozygous and so affect the health of the animal. Most of these genes have minor effects which gradually accumulate. There are many genes involved in traits like fertility, immune competence and mental stability. It is a fact that accumulation of homozygous recessive "bad" genes gradually diminishes the function of these systems no matter what.
Walkowitz & Wilcox 1994, Willis 1992, 1989, Onstott 1962 all view linebreeding and inbreeding as essentially the same and differing only in degree of intensity. Whether one considers inbreeding and linebreeding to be the same or feels they are two distinct breeding systems, quantifying the degree to which an animal is linebred (or inbred) provides important information regarding its potential genetic contribution.

If we traced our APBT pedigrees back far enough we would see that all our dogs are ultimately related and the inbreeding coefficient would be quite high. If such a dog were compared to a dog whose background is only known for a few generations, it would appear as if this dog were much less inbred. But this, of course, is not necessarily true. When we are comparing pedigrees, we must always consider for how many generations we are calculating the inbreeding coefficient.
Calculation of Inbreeding Coefficients will give an indication of how inbred a dog or a prospective cross is. Knowing these numbers enables the breeder to make choices that will reduce inbreeding.

A well educated opinion regarding inbreeding and modern homogeneity of dog breeds such as the APBT states that inbreedings/linebreedings time is past. This author indicated that if purebred dogs are to remain viable into the next century breeders need to rethink their strategy and work toward their goals with more emphasis on over-all health and concerted efforts to reduce the level of inbreeding in their breeds. Especially to me as a geneticist this is an important point considering three of the most prolific germ-lines in the show APBT and the health problems that are increasingly apparent.

Inbreeding and tight linebreeding typically act to reduces fertility, overall health and mental stability. Inbred animals are more prone to diseases such as infections and cancer, and more likely to be "highly strung" and nervous.

To understand why this happens we need to consider basic genetics:
All dogs carry some harmful genes. These genes are usually hidden because we have one good copy of the gene to compensate for the defective version and the good copy of the gene can perform the tasks required. If we double up on this gene through inbreeding/linebreeding we are removing the good copy and the harmful gene is the only one left. If this defective gene was the one that stabilizes our animal during stressful situations we end up with a nervous dog. If the defective gene was one that coded for genes that protect against cancer we have a dog less likely able to fight cancer. With many double defective genes the problem may be so severe that the animals die, suffer debilitating disease. Some are doubled up on defects in bone structure, reproductive anatomy, immune function, or may be blind, deaf, or plain sick all the time.

What we are seeing whether the breeder of a particular line acknowledges it or not is that when the overall homology (increased Inbreeding Coefficient?) of a specific germ line is focused by line or inbreeding there is an exacerbated tendency toward occurrence of disorders that are ultimately controlled by multiple sets of genes. Of particular interest cosmetically in the APBT are kinked tails, underbites and serious debilitating disorders as hip dysplasia, temperament issues (such as fear or human aggression), and congenital heart anomalies. Of utmost importance to an APBT breeder is prior knowledge of abnormalities that the common ancestors have produced. Continued linebreeding within such lines can promote the fixation of these defects permanently within the line resulting in extraordinary risk of genetic defects.

The most important issue is making health a top priority when linebreeding or inbreeding for trait selection. It is obvious even to those who promote inbreeding that screening for genetic diseases and not breeding affected individuals is important. As tests become available which will detect carriers of genetic problems, they should be put to use. However, before I state the following please do not for a second use the following words to justify or rationalize breeding selection of unhealthy animals. A particularly fine individual with a multitude of high quality traits that has carrier status for a detrimental defect should not automatically be precluded from breeding but the utmost care should be taken that they aren't bred to other carriers and more importantly those who buy puppies from a carrier parent damn well MUST be advised to screen the pup if they want to breed it. But eliminating proven carriers as breeding stock if they are anything but stellar otherwise is the only ethical approach. Defects such as dysplasia, temperament issues, congenital heart defects that are homozygous and expressed in a dog is however definitely a reason to spay/neuter. Definitely an animal that carries a trait is very different from an animal that displays the trait. (read the above again and note the difference between carrier and an animal that displays a recessive homozygous defect)

Inbreeding Depression
Research has shown that inbreeding depression, or diminished health and viability through inbreeding is directly related to the amount of detrimental recessive genes present. Some lines thrive with inbreeding, and some do not.

One of the effects of inbreeding is a decrease in the heterozygosity of important gene clusters such as those involved in immune expression. This effect places individuals and the population at a greater risk from homozygous recessive and immune related diseases. The impact of accumulating deleterious homozygous traits is called inbreeding depression. Ultimately this is considered to be a loss in progenies vigor due to loss in genetic variability or genetic options particularly those involving immune response.

Sometimes two different alleles may be better than one. Consider the major histocompatibility complex (MHC) of the immune system as indicated above. This large group of genes requires high diversity because each on of these individual genes adapts to combat a specific insult to the immune system. Having a variety of MHC alleles is vital to an animals ability to combat any given infection (for instance) or allergen. Thus, if both copies of the MHC is inherited from the same common ancestor due to inbreeding there is only half the combinations of genetic information to counteract environmental and harmful problems the immune system is geared to combat. Not only does this provide better defense against pathogens, but there is growing evidence that parents who carry different MHC haplotypes may have fewer fertility problems. This is not a universally accepted theory, but today one is hard pressed to find a conservation or zoo biologist concerned with preserving an endangered species who would not list maintaining maximum genetic diversity as one of his/her primary goals.

The MHC is very specific and in a way each gene in the MHC helps to create individual one type of cell that is specific to and capable of recognizing and killing only one kind of dangerous environmental problem. Examples of environmental insults includes such things as pollen, dust mite saliva, fly bite, insect venom, cancer cells, viruses, bacteria etc. The environment has billions upon billions of different insults that the animal may encounter in its lifetime and for each insult only one MHC gene will have the ability to counteract the insult.

Now consider a very inbred individual that has two copies of same MHC that followed the limits of probability and was passed down from the same ancestor. If both chromosomes of the dog have the identical immune system gene segments, that animal has lost half of its potential antibody genes. If that animal is further inbred, it starts to lose other individual gene segments to a genetic phenomenon we noted in a previous section called "crossover." Ultimately with continued inbreeding depression we may lose billions of potential antibodies. Maybe we lost that antibody that was required to fight that particular cancer our dog died of. Our dogs may also exhibit allergies to common things due to inbreeding depression. This is often seen when
Other signs of inbreeding depression are small litter sizes or difficulty producing or rearing young. Bitches from families that consistently produce small litters may be suffering inbreeding depression. Specific with lines of the APBT signs of inbreeding depression dog is indifferent to bitches in standing heat, incapable of mounting and successfully completing ties (hop off after a couple humps), bitches that require physically restrained, bitches that kill or damage puppies through intent or neglect and signify that this animal should not be bred.

breeders walk a tightrope between needing to reduce genetic variation to maintain uniform breed type and needing to maintain genetic diversity to avoid inbreeding depression. One of the most pervasive causes for inbreeding depression and decrease in genetic diversity is brought on voluntarily by breeders who continually breed the favored sires out to everyone that asks, while the majority of potential breeding males are never bred. The sire may not be of exceptional quality in all aspects or may be of exceptional quality. This bottleneck is made all the worse by the fact that the majority of breeding bitches are often sired by other popular studs from previous generations.

It is possible to prevent this deterioration in health and vigor by limiting linebreeding and by selecting rigorously for highly fertile, vigorous, long lived, mentally stable animals. Responsible, intelligent dog breeders do just that. Only breed the best to the best and cull the rest.
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