A great article I found and wanted to share:
Dr. B, What is a Polygenetic trait?
Polygenetic traits: A number of liability genes must combine to cross a threshold and produce affected individual. Sire and dam carry a genetic load of liability genes. Some are good such as mothering, litter size and milking. Polygenetic genes are difficult to select away from such as Hip Dysplasia, PRA, bone joint and hernia's.
Search widely in pedigree to see if this condition exists with increase frequency in your line. Even if we do not know the inheritance, we can select away from the issue. Your goal is always to maintain the line and each generation to have fewer “bad genes” than the last. By selecting less affected offspring and replacing more affected parents we remove the gene Phenotype and Genotype.
In the past we tried not to breed the line affected but that was wrong as we eliminate good genes. Maintain genetic diversity of your population is essential for progress. If you just outbreed without working to eliminate the genotype you will fail. Soon you deal with it again the next generation. You just disseminate detrimental recessive genes in the line.
What to do: Ask about your dogs you have sold and know what issues have arose. It is true environment plays a role but will only show up if you have the genes for it. Poor nutrition can alter expression of musculoskeletal disorders(Hip, HOD, Legg Perthes and Knees) especially in the growth phase. Knowing you have the gene and what they are is powerful.
Look widely in the pedigree. Knowing the siblings are free of issues is more important than the depth of pedigree! Genetic test where appropriate. It is once in a lifetime and the only way to know your breeding program is on the right track.
Our goal is always to maintain the line and each generation to have fewer “bad genes” than the last. Look for an article in the near future on Genetics.....Dr. B