[size=12]It has been a while since we had an intelletcual discussion about genetics or breeding practices in general, so I am posting this snippet from an article that is also available in the content section of this site. So, please read this carefully and make a comment with your opinion.
[quote="Vargas, Cargill, Coile"]
INBREEDING AND INBREEDING DEPRESSION: You can't fool Mother Nature
Evolution is thought to be a gradual change in the kind and frequencies of alleles. Those mutants that are harmful are either eliminated or kept at low frequencies by natural selection. However; with artificial selection , especially when a breed is being developed, it is the individuals that exhibit the greatest expression of the desired traits that are chosen to breed the subsequent generations. When only a few dogs are used to produce the next generation, a high proportion of their genes will be in the next population of potential breeding animals. When these related dogs are then interbred, the chances of them passing on the same allele that they both received from their sire and dam is 25%. Thus, inbreeding increases the chance that subsequent offspring will carry identical copies of the same allele (be homozygous at that locus). Increasing inbreeding increases the chance of homozygosity and can lead to the loss of one of the alleles from the population.
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, which results from homozygosity of deleterious alleles. The majority of alleles detrimental to life and reproduction tend to be recessive, for the simple reason that if they were dominant, they would have been expressed in the individual's phenotype, and that individual would have been less likely to reproduce. If recessive, only those individuals with homozygous recessive alleles would be reproductively compromised; heterozygotes would be unaffected. Every dog (and every human) carries deleterious recessive alleles, so the chances of the foundation stock carrying them is virtually 100%. If very few dogs were used as foundation stock, their progeny would have to be interbred, and in only a few generations all of the dogs would be closely related. Breeding closely related dogs is inbreeding. An inbred dog has a greater likelihood of receiving the same allele from both its sire and dam, and thus a greater likelihood of being homozygous for a deleterious trait. In an inbred population, as long as the animals can still reproduce, this homozygosity can become fixed in the population due to the chance loss of the other allele. What this means for the breeder is that too great a reliance on inbreeding will lead to loss of 'fitness', i.e., failure to reproduce. Fewer litters are produced, the number of whelps will decrease and those that are born will fail to thrive. Taken to extreme, the effective breeding population could be so diminished that the breed would face extinction. [/quote][/size]