There are three variables involved in canine coat types: the length of the coat, the presence of furnishings, and the presence of curly hair. Hair curl is a dominant characteristic caused by a mutation in the KRT71 gene. This gene codes for keratin, a protein that determines the type of hair.
This mutation is fixed in some breeds, such as the Irish Water Dog. This mutation is, however, variant in other breeds, such as the Kuvasz. The hair curl mutation can also be accompanied by the other mutations that can change coat length and type. For example, the Airedale Terrier has both the curly coat and furnishings that are responsible for their trademark eyelashes and mustaches. Other breeds, such as the Standard Poodle, can have all three mutations, creating a long-haired curly coat with furnishings.
Because the hair curl gene is dominant, a dog only needs to have one copy of the gene to express that phenotype. This can appear as either C/c or C/C. Hair curl would not appear in dogs whose gene codes as recessive (c/c). A dog can carry the allele responsible for non-curly hair, and could pass the recessive allele on to any offspring. If two dogs that are both carriers of the non-curl gene are bred (C/c), there is a 25% chance per puppy that they will inherit the recessive allele (c/c), resulting in a dog with non-curly hair.