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OCD Bullyologist
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Removing Defects
So Which Is Best?
No simple answer will determine which breeding strategy is best. Whether you use Inbreeding, Linebreeding, Outcrossing or a combination of all three it all boils down to philosophy. Each strategy has favorable and adverse consequences. Using a combination of strategies and knowing how they drive selection and breeding in various portions of the population can help you choose among them, depending on the goals of your breeding program.
Inbreeding (and to a lesser extent linebreeding) makes for more consistent and more predictable animals, which can be good in some situations. It is useful if selection for vigor is going to be possible. Inbred populations have little variation, so that performance (temperament, conformation, color) can be accurately predicted. Inbreeding can also bring recessive defects to light. This too can be either good or bad. It is bad if selection is not going to remove (or at least identify) carriers from the population. It is good if the identification of carriers is going to act to reduce their frequency in the population.
With outbreeding, vigor goes up, especially reproductive vigor. Uniformity generally goes down, although one notable exception is the first cross between inbred or linebred animals that are from different tines. Crossing inbred lines usually generates very uniform animals, but these uniform animals do not in their own turn produce uniform offspring.
Outbreeding also tends to decrease (at least initially) the chance that rare recessive genes are brought to light. The good news is that many diseases are probably due to rare recessive genes, and therefore outbreeding is one way to avoid their expression. The bad news is that they will eventually show up in a population, for carriers eventually become common enough that outbreeding pairs them up and the diseases or deformities are expressed. In a deliberately outbred population the expression of defects can indicate that the genes responsible (for those defects that are genetic in origin) are widely dispersed throughout the entire population.
One good option is to breed for outbred females and linebred males. A single strategy to accomplish this is somewhat tricky but possible. In this system it is essential that the linebred males come from carefully documented lines, and that they are not carriers of any deleterious genes. Not just any male will do!
THE SURVIVAL OF THE FITTEST
This preservation of favourable individual differences and variations, and the destruction of those which are injurious, have been called Natural Selection, or the Survival of the Fittest. Man can act only on external and visible characters: Nature, is allowed to personify the natural preservation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good: Nature only for that of the being which she tends.
Natural Selection Is the Key
Selection is the force that allows reproduction of some individuals and not others. It operates independently of any type of breeding system in animal populations. Selection is therefore a force for change in the overall genetic makeup of a population. Selection is a powerful tool, one that can irreversibly change a population.
Selection can involve any trait whatever: size, color, temperament and conformation. Selection can be intense and cause fairly rapid change over a few generations, or it can be more relaxed and change the population more slowly in the desired direction. Because selection can irreversibly change a population, the breeder needs to carefully consider his or her goals.
Selection can be responsible for changing the incidence of recessive genes. If a defect or disease is due to a recessive gene and the defect can be treated, then it is possible for the defective animals to reproduce. All offspring of these animals will carry the gene for the defect, whether or not they actually express it. This transmission, repeated in many individuals, can act to increase the frequency of genes for defects. Other alternative plans have different consequences.
Limiting reproduction of known carriers is important for the long-term genetic health of the population, although its practice will always be unpopular with owners of otherwise-outstanding individual breeding animals that happen to be carriers of genetic defects. The widespread use of carriers ensures that when these dogs are bred and carry on these unknown recessive genes, they will infect many many dogs in the population. The difficulty is that, if left unchecked, the genes can become so common in a population that selection becomes a difficult pill to swallow, because then a high number of individuals must be removed from reproduction. Some of the carriers are bound to be otherwise exceptional, and these are the animals for which the choices become very difficult.
Identification of carriers can come about in different ways. One way is to simply let individual breeding practices eventually bring carriers to light. This works reasonably well for defects of low incidence, since they are unlikely to overwhelm the population. The danger of this approach is that a single undetected carrier sire that is used widely can spread the defective gene far and wide before it is detected. Once these genes become common, reducing their incidence is a real headache.
In the case of more common or severe defects, it is possible to test for carriers more deliberately. One of the most powerful tests for genetic defects is the mating of parent to offspring. If anything weak is present it will be exposed. Unfortunately, the number of matings needed for this type of test is relatively high. To be 95 percent sure that the animal is not carrying deleterious recessives, it takes twenty-three normal offspring from daughters. To be 99 percent sure, it takes thirtyfive normal offspring. Obviously, any abnormal offspring produced at any point along the way implicates the sire as having the genes for that defect. The logic works only if the defects are genetic.
If a carrier is detected, by whatever means, then the next step needs to be pondered carefully. If selection is aimed at decreasing the number of carriers, many different routes can be taken. One method is to neuter the affected individuals as they become known, the parents of the defective individual, and all of their previous offspring. This is the most radical selection against a defect, and it effectively removes carriers from the population as they are detected as well as some noncarriers simply because, based on the law of averages, they are more likely to be carriers by virtue of their relationship to known carriers. At very low gene frequencies carriers are unlikely to be detected because they are unlikely to be mated to another (equally rare) carrier. So while the "neuter all carriers" approach will work to dramatically reduce the number of carriers in a population, it rarely completely eliminates all carriers since some slip through the cracks of the system.
Other selection plans that work against carriers are better than nothing, but less drastic than neutering all carriers. One such plan is to neuter the sire because he can spread the gene more widely than can the dam, which produces fewer offspring. Still, half of the offspring of the carrier dam will be carriers. One approach to this problem is to neuter all of her sons but allow her daughters to reproduce. About half of these will be carriers. If these are in turn used for reproduction, the carrier rate goes down to about one-fourth, although which specific fourth is uncertain without a breeding test. If excellent males are generated, an alternative to this scheme would be to test-mate them to known carrier females to determine which of the males do not carry the defective gene. Those documented as free of the gene can then be used widely and safely for breeding of animals free of the specific defect. In this way, the positive traits of the line can be continued while leaving behind the defect. The process is long and involved but well worth the effort in some circumstances.



From Dr. Jerrod Bell
 
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