Canine hipdysplasia is a quantitative trait that continues to affect most large breeds of dogs in spite of approximately 40 years of effort to reduce the impact of this condition. Breeders and veterinarians rely on radiographic and universal registries to facilitate the development of breeding schemes for reducing its incidence. Data [Wood and Lakhani \((2003) .\) Vet Rec. \(152: 69-72\) indicate that there is a "month-of-birth" effect on hip dysplasia in Labrador retrievers and Gordon setters, whereby the frequency and extent of expression of this disorder vary depending on the time of year dogs are born. Speculate on how breed- ers attempt to "select" out this disorder and what the month-of-birth phenomenon indicates about the expression of polyzenic traits.

Hip dysplasia is a quantitative trait that affects the hip joints in dogs, especially larger breeds. A quantitative trait is a characteristic influenced by multiple genes, also known as polygenic traits. These traits usually result from the combined expression of several genes, and their phenotypes often follow a continuous distribution.

According to the data provided, there is a month-of-birth effect on hip dysplasia in Labrador retrievers and Gordon setters. This means that the frequency and extent of expression of this disorder vary depending on the time of year dogs are born. This could be due to seasonal changes affecting gene expression or environmental factors that influence the development of dogs born at different times of the year.

Breeders attempt to select out disorders like hip dysplasia by carefully choosing the dogs used in their breeding programs. They evaluate the dog's pedigree, history of hip dysplasia, and appearance of hip joints through radiographic imaging (X-rays) to determine the potential risk of passing on the traits that cause hip dysplasia. Breeders may also use registries to track the incidence of hip dysplasia for better-informed breeding decisions and to identify breeding dogs with lower incidences of hip dysplasia.

The month-of-birth phenomenon indicates that the expression of polygenic traits like hip dysplasia can be influenced by environmental factors and conditions experienced by the dog during its development. These factors may include temperature, nutrition, or hormone levels. The month-of-birth effect suggests that breeding strategies should consider the interaction between genetic and environmental factors that could affect the expression of polygenic traits, such as mating dogs with different birth months to minimize the environmental impact on the expression of hip dysplasia. In conclusion, breeders attempt to select out disorders like hip dysplasia through careful breeding selection and by considering the interaction of genetic and environmental factors affecting the expression of polygenic traits. The month-of-birth phenomenon indicates the importance of understanding not only the genetic basis for traits like hip dysplasia but also the environmental factors that can influence their expression.