POLYGENIC  These traits are more complex than the typical dominant or recessive genetic trait. The additive interaction of the genes can cause variable results and the gene can be easily passed on to other generations without being identified.  Canine hip dysplasia and elbow dysplasia are believed to be polygenic defects.  1. As with the recessive trait, both the sire and the dam must contribute one or more of the genes that cause the abnormal phenotype in the offspring.  2. Unlike recessive traits, the contribution from the sire and dam need not to be equal.  3. Since we do not know the number or the specific effect of the genes involved in polygenic traits in dogs, no predictable Mendelian ratios are associated with these traits.  4. Both sexes are affected with polygenic traits (excluding sex-limited traits), but not necessarily in equal numbers.  5. The trait may skip generations and may appear to be erratic in occurrence.  Especially when it comes to polygenic defects it is hard to tell which one of the parents is mainly responsible for the defect in part of the offspring. It is throughout possible that the bitch and the sire are equally responsible, but it could be that the bitch’s part is 99,5% and the sire’s part is 0,5%, or the other way around. A breeder might exclude both animals from the breeding stock, but this could mean that he would exclude a valuable animal that would produce normal puppies if mated to another partner. 

Homozygous & Heterozygous Genes. Heterozygous gene pair example: Tt (members are dissimilar) Homozygous gene pair examples: TT and tt (members are alike) Breeders goal: To "arrange" good genes in homozygous pairs to ensure that no matter which member of a gene pair a parent happens to pass on to a puppy, it will be a "desirable" gene.