Genetic Load
One of the most fundamental misconceptions is that most of the individuals in a normal population do not carry genes for genetic diseases. Starting with this assumption, many breeders argue that it should be possible to breed animals with a desired conformation while avoiding undesirable traits. They will concede that some unfortunate individuals do carry recessive genes for these traits, but believe that if they choose their breeding stock carefully, they can avoid the problems that others are having. If problems do show up, it is due to "bad luck", the lack of direct genetic tests for recessive genes, or because another breeder has been concealing something.
The truth is that it is virtually impossible to avoid genetic disease. Geneticists believe that most species carry a "genetic load" of 3-5 recessive lethal genes. The difference between purebred dogs and a human is that the latter have something in excess of 2500 genetic diseases, but most of them are extremely rare and thus seldom come from both parents to produce an affected child, whereas many dog breeds have a relatively small number of very common genetic diseases.
Wooden Sculpture of Science Genetics
Genetics
Bloom'sIt is the frequency of these problems, rather than the number of different ones, that is the true indicator of genetic health in a population.
When direct genetic tests do become generally available, they will be used to identify the defective genes carried by a dog that may be bred not for the purpose of eliminating these individuals from the gene pool, but for planning crosses wisely so that the defects don't match up to create a homozygous affected individual. They will also make it easier to identify individuals carrying dominant traits that are not always expressed, and those who are homozygous for late onset diseases, such as SA, which do...