Certain canine defects are inherited in a more complex manner by the joint action of many genes together with environmental influences. Hip dysplasia, a defect also known in man, cattle and cats, is common in the dog. An affected dog will be born with apparently normal hips, but as the dog becomes older the ball and socket joint of the hip ceases to fit properly. The inherited defect may vary from a mild case with no obvious ill effects to severe crippling.
The variation seen in hip status is controlled by many genes which act in an additive fashion: the more hip dysplasia genes a dog has, the worse will be its hip status. However, the action of some genes depends more on combinations of specific genes than on numbers, and environmental factors can have an adverse effect upon hip status. Rapid growth in early life and certain forms of exercise, in particular high jumping or scaling, can accelerate hip dysplasia. The condition is more prevalent in large breeds of dog and largely unknown in tiny breeds. However there are great differences between breeds, and deliberate attempts have been made to select against the condition. Estimates of the additive genetic component vary from 25 to 45 percent, but they depend on the method of assessment, the breed and the country of study. In some European countries, notably Scandinavia and Switzerland, compulsory schemes have been introduced to fight hip dysplasia, while optional schemes exist in the united states and Britain. Methods of assessment rely upon X-rays of the hips taken after the age of one year ( two years in the United States). In some countries, hips are graded, usually into five different categories; the British Veterinary association scores hip status numerically on a scale from 0 to 54, with lower scores being better.