<i>FGF5</i> Allelic Heterogeneity in Long‐Haired Belgian Shepherd Dogs

The Belgian Shepherd dog breed comprises four varieties distinguished primarily by coat type and coloration, with hair length largely controlled by allelic variation in the FGF5 gene. While five long-hair alleles (l1-l5) have been described across dog breeds, the long-haired phenotype in Belgian Shepherds has been linked to the l1 allele. In this study, sequencing of a long-haired female from a Malinois breeding revealed compound heterozygosity for the l1 and l4 FGF5 alleles. Subsequent screening of archival DNA samples (n = 240) identified additional l4 carriers in Malinois and Tervueren, including further l1/l4 individuals. Because the l4 allele had previously been reported only in Afghan Hound and Eurasier, these findings suggest historical introgression or shared ancestral variation. This work expands the known breed distribution of the l4 allele and has practical implications for genetic testing and breeding strategies.