Beyond Appearances: The Hidden Coat Diversity of the Sicilian Mastiff Revealed by Integrated Phenotypic and Genetic Analyses

Coat colour is one of the most recognizable and traditionally selected traits in dog breeds. However, despite its apparent simplicity, it represents a biologically complex phenotype, shaped by the interaction of multiple genetic loci, modifier genes, and variable phenotypic expression, challenging standardized phenotypic classification. This study presents the first characterization of coat colour and texture in the Sicilian Mastiff, integrating pedigree records, photographic assessment, and genetic analyses. Breeder-declared pedigree coat colour was examined for 760 dogs to evaluate terminology and its consistency, while photographs from 197 individuals enabled standardized phenotypic classification. Fifty dogs, selected within a monitored subset, were genotyped for 40 coat-related variants using the MyDogDNA™ commercial test, with additional analysis of KRT71 curly-related variants in nine animals. Pedigree records included 115 terms for only 23 colour categories, revealing substantial inconsistency in terminology. Photographic evaluation identified considerably greater phenotypic diversity than registry data, showing that complex traits like brindling, ticking, melanistic masks, and white markings were frequently omitted or simplified; for example, only 8/64 Brindle black and tan dogs were identified as such in the registry. Genetic analyses confirmed segregation of alleles influencing eumelanin/pheomelanin distribution, dilution, white spotting, and hair texture—including both KRT71 variants contributing to the characteristic curly coat of the breed—, while no known variants associated with merle, harlequin, cocoa, or albinism were detected. Our findings could provide valuable support to the breed association in requesting that the Italian Kennel Club (ENCI) revise the breed standard terminology and coat colour recording process, including allowing updates during growth, to improve clarity and consistency. Finally, requiring photographic documentation for registration applications could improve the reliability of coat colour assessment.