such as a poor conformational fault or a disease. Some closed stud books, particularly for certain European breeds such as the Finnhorse and the Trakehner, may also have a set of studbook selection criteria where animals must meet either a conformation standard, a performance standard, or both. Open In an open stud book, animals may be registered even if their parents or earlier ancestors were not previously registered with that particular entity. Usually an open stud book has strict studbook selection criteria that require an animal to meet a certain standard of conformation, performance or both. This allows breeders to modify breeds by including individuals who conform to the breed standard but are of outside origin. Some horse breeds allow crossbreds who meet specific criteria to be registered. One example is the semi-open stud book of the American Quarter Horse, which still accepts horses of Thoroughbred breeding, particularly via its appendix registry. Among dogs, an example of an open stud book would be the registries maintained by the American Kennel Club as its Foundation Stock Service. In some cases, an open stud book may eventually become closed once the breed type is deemed to be fully set. In some agricultural breeds, an otherwise closed registry includes a grading up route for the incorporation of cross-bred animals. Often such incorporation is limited to females, with the progeny only being accepted as full pedigree animals after several generations of breeding to full-blood males. Such mechanisms may also allow the incorporation of purebred animals descended from unregistered stock or of uncertain parentage. More controversial open stud books are those where there are few, if any qualifications for animals other than a single trait, such as a "color breed," particularly when the color is not a true-breeding characteristic. However, some breeds have a standard color or color preference that is one criterion among others used to register animals. Appendix registries Some open or partly open registries may permit animals who have some but not all qualifications for full registration to nonetheless be entered in a preliminary recording system often called an "appendix" registry. The most notable is that of the American Quarter Horse Association, which allows part-Thoroughbred/part-Quarter Horse foals to be recorded and shown, with full registration allowed after the horse achieves a set performance or merit standard akin to that of a merit registry. Other appendix registries are seen in certain color breeds