You say you don't understand what hip dysplasia is or how a dog acquires it?  Well, don't feel too badly, you are not alone!  Even the experts find this disease to be very baffling! The following is an excerpt from an article published by Cornell University. -- The absence of hip dysplasia in canine parents guarantees dysplasia-free pups. Unfortunately, out of 100 matings of "normal" dogs in breeds affected by hip dysplasia, 75 percent of puppies will be "normal" but 25 percent, on average, will have hip dysplasia. Genes for hip dysplasia are believed to be "masked" or hidden in some generations, making the elimination of the disease from breeding stock even more difficult. The following is an excerpt from an article published by Atlantic States ...Both heredity and environmental factors are important in the development of CHD. CHD has an estimated inheritability that ranges from 0.2 to 0.6, with 0.0 being non-inherited condition and 1.0 meaning that a condition is completely under genetic control. Thus, demonstrating that CHD is a condition where both the environment and the genetics of the dog play a role in the development of this disease.The interaction between the dog's genetic makeup and environment determine whether an individual dog will develop CHD. The genetics of the dog, to a large part, determine if a given dog has the potential to allow the environmental factors to act in such a way that CHD is produced. However, even dogs that are not genetically predisposed to develop CHD can contract the disease if they are pushed too hard when young by hyper-nutrition and excessive exercise.....It is entirely possible for a dog with an OFA rating of excellent to produce puppies that develop CHD. How can this occur?Without going too deeply into the genetics, some basic genetic background is needed to explain how an OFA certified animal can produce off-spring that develop CHD. The following is a gross over simplification of the genetics of the CHD; but does provide a framework for understanding a problem with multiple gene control over a single trait. Dogs have two copies of each gene that controls CHD (the total number of genes that control or contribute to the development of CHD is unknown; but it is believed that many are involved). They get one gene from their mother and the other from their father. Some of the "good genes" can mask the presence of a "bad gene". In other words some of the "good genes" are dominant over the recessive "bad genes". Thus, the physical expression of the "good genes" will be result in good hips, even though the animal is carrying a number of hidden (recessive) "bad genes". When the sperm or egg is formed the two copies of each gene are separated so that sperm and the egg end up with only one copy of each gene. In a case where an animal had a copy of one good gene and one copy of the bad gene, 50% of the eggs or sperm would get the "good gene" and 50% will get the "bad gene". Now recall that many genes are involved in the development of CHD, so this separation occurs for each of the genes that control hip joint development. Thus, a very few of the eggs and sperm will get all of the "good genes" and a very few will get all of the "bad genes" but most will get a mixture of both "good" and "bad" genes that control CHD. The outcome of a particular mating will be determined by the match up between the genetic make -up of the parents. If the bad recessive genes line up incorrectly then more of the puppies could have hip dysplasia. However, if the good dominate genes line up well then many of the pups will have good hips, but may still carry the hidden bad genes. Complicating the situation even further is that some genes may be good when combined with one set of genes and bad when combined with different set of genes. It is also possible for some genes to have a greater or lesser influence on CHD than others, adding to the complexity of the problem. Thus, one can see that the genetics of CHD is very complicated.Some puppies will have the same genetic combination as the parents. Some may have a more desirable genetic make-up than either parent; and others may have a less desirable genotype. Predictions of a specific CHD outcome from a particular mating is currently very difficult if not impossible.However, it is OFA's opinion, that a dog with fair hips and less than 25% of its littermates exhibiting dysplasia is a better breeding prospect than a dog with excellent hips having more than 25% of its brothers and sisters affected by hip dysplasia. In other words, less of the bad genes are likely to be present where a smaller percentage of the littermates have CHD. Ideally, we would want to achieve a homogeneous gene pool composition where the dogs carry two copies of the good genes for each gene that controls CHD. In other words eliminate the bad hidden (recessive) genes from the pool. This could be achieved by careful line breeding, making good hips part of that line's type. However, to accomplish this it is important to look beyond the individual animals of the proposed mating and consider the OFA status of the littermates as well as other relatives on both sides of the mating before proceeding. It is also important to point out that a good dog is more than just good hips. Care must be taken that the breeding of dogs is not based only on one trait. Otherwise we may end up with dogs that have excellent hips but have poor temperaments, bad coats, no herding instincts, or other genetically related health problems. The whole animal should be considered with hips status being important but not the only considerationFrom this article it is easy to see why CHD is such a difficult problem to control. However, through careful breeding selections of OFA certified animals that have a low incidence of CHD in their littermates and in their pedigrees, we should be able to reduce, although not eliminate, CHD as a problem.