A dysplastic hip joint is too shallow. In a dysplastic hip joint the pelvic socket is shallow, and so the head of the femur rides in and out of the socket. Due to the shallowness of the hip joint, the animal with HD will often, though not always, have more of a swaying movement in the rear. This constant movement wears down the cartilage that normally overlies the bony surfaces of the joint, sooner or later causing bone to rub on bone.
Extra bone is laid down around the edges of the pelvic socket.
An animal with these changes would be said to have degenerative joint disease, or DJD in the hips. We don't yet have all the answers to this question. The exact way hip dysplasia is transmitted genetically is still being investigated. Very roughly speaking, if both parents of a puppy have HD, the pup will almost certainly have it as well. Among the breeds currently most likely to have HD are the Golden Retriever, Labrador Retriever, and Rottweiler.