Question

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.

Short answer

Answer: Breeders attempt to select out hip dysplasia by carefully choosing dogs in their breeding programs based on pedigree, history of hip dysplasia, and appearance of hip joints through radiographic imaging. They may also use registries to track the incidence of the disorder for better-informed breeding decisions. The month-of-birth phenomenon indicates that the expression of polygenic traits like hip dysplasia can be influenced by environmental factors experienced by the dog during its development, emphasizing the importance of considering the interaction between genetic and environmental factors in breeding strategies.

Step-by-step solution

Understand Hip Dysplasia and Quantitative 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.

Month-of-Birth Effect

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.

Breeding Selection Procedures

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.

Month-of-Birth Phenomenon and Polygenic Traits

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.

Key concepts

Quantitative Traits

Quantitative traits are characteristics that are not just influenced by one single gene, but rather by multiple genes working together. These traits can be measured and usually exhibit a range of phenotypes rather than a simple present or absent status.

In dogs, hip dysplasia serves as an example of a quantitative trait because it's determined by the collaborative impact of many genetic factors. Unlike single-gene disorders, the complexity of quantitative traits makes predicting and controlling their expression more challenging for breeders. It also means that pure statistical methods, like the calculation of heritability estimates, are used to understand how much of the variation in hip dysplasia is due to genetics as opposed to environmental factors.

Polygenic Traits

Polygenic traits are a type of quantitative trait controlled by several genes, each contributing a small amount to the overall phenotype. These traits often show a bell-shaped distribution in a population, indicating a range of variation among individuals.

Hip dysplasia, as a polygenic trait in canines, is affected by multiple genes that influence the development and strength of the hip joint. Because multiple genes are involved, selective breeding to reduce the risk of this condition requires detailed records and understanding of genetic inheritance, looking beyond simple patterns that govern single-gene traits.

Breeding Selection Procedures

Breeding selection procedures involve deliberate choices by breeders to mate specific individuals with desired traits to produce offspring that are more likely to exhibit those traits. In the context of hip dysplasia, breeders might use pedigree analysis, hip scoring systems, and genetic testing to inform their breeding strategies.

The goal is to reduce the incidence of hip dysplasia by selecting against the genes that contribute to its development. Responsible breeders will prioritize the health and genetic diversity of the breed, avoiding breeding from dogs that have shown signs of hip dysplasia, and aiming to preserve or improve upon the anatomical structure and health of the hips in future generations of dogs.

Environmental Impact on Gene Expression

The interplay between genetics and environment can profoundly influence the expression of polygenic traits. For instance, the 'month-of-birth' finding in the exercise suggests that when a dog is born may affect the expression of hip dysplasia due to exposure to different environmental conditions like varying temperatures or nutrition availability during crucial developmental stages.

Breeders might consider these environmental impacts and may adjust their breeding schedules or the care provided to pregnant bitches and young puppies to minimize negative influences on their development. Understanding how the environment can modify gene expression is essential for comprehensive breeding strategies that aim to reduce the impact of polygenic diseases like hip dysplasia.