Influence of environment: Environment can play several roles in influencing the expression of some genes. In hip dysplasia, for instance, environment can influence how dysplastic a dog becomes. A dog cannot become dysplastic unless it has a majority of "bad" hip genes. However, when a dog does have a majority of bad genes, and it is kept overweight or allowed to grow too fast, or is exercised too much or too little, that dog will end up with a worse case of hip dysplasia than it otherwise would have had.