Coat Length Genetics in Dogs: The L Locus Explained

Coat length in dogs — whether a dog has a short, dense coat or a long, flowing one — is largely determined by the L locus, which sits on the FGF5 gene. This is one of the simpler genetic loci to understand, and testing for it is especially important in breeds where both coat lengths are common.

The L Locus Alleles

L (short coat): The L allele produces a normal, shorter coat. It is dominant over the long-coat allele.

l (long coat): The l allele, when present in two copies (l/l), produces a longer coat. One copy (L/l) produces a short-coated dog that carries the long-coat gene.

A dog must be l/l (homozygous recessive) to express a long coat. Any dog with at least one L allele will have a short coat, regardless of how many l alleles it carries.

Why This Matters for Breeders

In breeds like German Shepherds, Australian Cattle Dogs, and Chihuahuas, both coat lengths exist within the breed and are genetically distinguishable. A short-coated German Shepherd can be L/l — meaning it carries one long-coat allele. Bred to another L/l carrier, 25 percent of puppies will be l/l (long-coated).

In some breeds, the long-coat version commands a premium (long-coat Chihuahuas, for example). In others (competitive German Shepherds), long coat can be a disqualification for show. Knowing your stud's L locus genotype helps buyers know exactly what to expect.

Multiple FGF5 Mutations

Recent research has identified multiple independent mutations within the FGF5 gene that all produce long coat. This means a dog can be "long coat" for different underlying reasons — and two long-coat dogs that carry different mutations may produce short-coated puppies. Comprehensive DNA panels test for all known variants.

Breeds Where L Locus Testing Is Valuable