A shortened and kinked tail—which is referred to as a “screw tail” — is one of the distinctive morphological traits that characterizes Bulldogs, French Bulldogs and Boston Terriers, which were historically developed from the Bulldog breed and, thus, closely related breeds. The ‘screw tail’ is caused by a variety of malformed and fused vertebrae and lack of approximately 8 to 15 caudal vertebrae.n addition to the Dvl2 frameshift mutation, the three Bulldog related breeds harbor other mutations in developmentally important genes that could affect their craniofacial morphology.