Originating from dogs bred and used by the Reverend John Russell in the early 19th century, from whom the breed takes its name, the Jack Russell has similar origins to the modern Fox terrier. In 2009, there were 1073 Jack Russells registered with the ANKC, compared to 18 for the Parson Russell terrier. Other modern breeds are often mistaken for modern Jack Russell terriers, including their cousin the Parson Russell terrier, the Tenterfield terrier, and the Rat Terrier. Some are prone to dislocation of the kneecaps, inherited eye diseases, deafness and Legg Perthes—a disease of the hip joints of small dog breeds. Prone to mast cell tumors. Legg–Calvé–Perthes syndrome, also called Avascular Necrosis of the Femoral Head, is where the ball section of the femur in the hip joint deteriorates following interruption of the blood flow and is the same condition as in humans.