From the Canine Genetics List with permission: Jacque (another listmember ) wrote: Please check out this site: http://www.ukcdogs.com/cfarticles/cf62599-2.html I'd like your comments on this.....especially I would like to know if and/or how inbreeding depression has been avoided. Dr. Armstrong replied: With a few possible exceptions that I will deal with in a minute, all members of all populations of all animals carry a genetic load of about 3-5 lethal equivalents (ie. mutations that would kill if homozygous). The trouble lies in the "equivalents".  One LE may actually be 10 genes that reduce your genetic (reproductive) fitness each by 10%.  There appears to be a continuous distribution from 100% lethal to < 1% lethal and maybe 100+ genes involved.  If we have 35,000 genes, even if there were 350 detrimental genes, that would only be 1%.  When a dog (cat, mouse, human) is bred, the progeny will, on average, get 50% of the father's load and 50% of the mothers. If two individuals are selected from a fairly large population, few if any of these genes will actually match up, and even if they do, most cause only a minor loss of fitness and are not noticed. If we breed close relatives, they are more likely to have the same set of  "underachievers", and will therefore be much more likely to become homozygous for some.  The worst may kill very early and thus tend to eliminate themselves, and the other obvious ones would hopefully be noted by the breeder and not used to continue the line.  The most dangerous are those that reduce fitness by 5-10%.  Most will not be detected and we may even be selecting for some that are linked to traits we like. At some point someone notes that bitches are having whelping problems, litters appear to be smaller, etc., but the majority will attribute it to contaminants in the food, pollutants in the water and the like. These other factors may play a role, but many of them are likely magnified by the reduced fitness of the individuals. How do the lab mouse breeders "get away with it" -- by starting many, many lines and watching most of them die out as they become more inbred.  If you breed enough lines, a few may make it through without any major health problems, but I would be surprised if they were as fit as their wild counterparts. (They may be healthier due to better care and nutrition, but I doubt they'd do as well in the wild.)  I think cheetahs are in much the same situation.  They made it through some event 10-20,000 years that wiped out the North American and other populations, and the African population was lucky.  Conservation biologists still worry about their adaptability. It's not that it can't be done.  It's just that no one has the resources to start several 100 inbred lines of Poodle, Aussie, GSD, or whatever, or wants to put down hundreds if not thousands of sick puppies.  The lab mouse has the advantage of a shorter generation time and less emotional attachment. In some ways, though, the breeding of purebred dogs is akin to this, except that the lines are our breeds, most of which are becoming progressively more inbred. My observation is that most are on the road to extinction,  but most breeders do not even realize they are part of an experiment. John Dr. John Armstrong University of Ottawa http://www.magma.ca/~merlin