usually L/L or L/l, which does not match with their long-haired phenotype. The Yorkshire and Silky Terriers share common ancestry and likely share an unidentified gene responsible for their long hair. The Afghan Hound has a unique patterned coat that is long with short patches on the chest, face, back and tail. The Irish Water Spaniel may share the same pattern gene, although unlike the Afghan Hound, the IWS is otherwise genetically a long-haired (fixed for l/l) breed. Other related genes Hairlessness gene Some breeds of dog do not grow hair on parts of their bodies and may be referred to as hairless. Examples of hairless dogs are the Xoloitzcuintli (Mexican Hairless Dog), the Peruvian Inca Orchid (Peruvian Hairless Dog) and the Chinese Crested. Research suggests that hairlessness is caused by a dominant allele of the forkhead box transcription factor (FOXI3) gene, which is homozygous lethal. There are coated homozygous dogs in all hairless breeds, because this type of inheritance prevents the coat type from breeding true. The hairlessness gene permits hair growth on the head, legs and tail. Hair is sparse on the body, but present and typically enhanced by shaving, at least in the Chinese Crested, whose coat type is shaggy (long + wire). Teeth can be affected as well, and hairless dogs have sometimes incomplete dentition. It is one of the things which become better the last years, as it is common to select healthy dogs with good teeth for breeding. The American Hairless Terrier is unrelated to the other hairless breeds and displays a different hairlessness gene. Unlike the other hairless breeds, the AHT is born fully coated, and loses its hair within a few months. The AHT gene, serum/glucocorticoid regulated kinase family member 3 gene (SGK3), is recessive and does not result in missing teeth. Because the breed is new and rare, outcrossing to the parent breed (the Rat Terrier) is permitted to increase genetic diversity. These crosses are fully coated and heterozygous for AHT-hairlessness. Ridgeback Some breeds (e.g., Rhodesian Ridgeback, Thai Ridgeback) have an area of hair along the spine between the withers and hips that leans in the opposite direction (cranially) to the surrounding coat. The ridge is caused by a duplication of several genes (FGF3, FGF4, FGF 19, ORAOV1 and sometimes SNP), and ridge is dominant to non-ridged. Long Hair There are many genes and alleles that cause long hair in dogs,