A method of determining susceptibility to periodontitis in a Shih Tzu dog, Yorkshire Terrier dog or a dog of a breed that is genetically related to the Shih Tzu or Yorkshire Terrier breed, comprises: a) typing the nucleotide present in the genome of the dog at or at a position equivalent to each of the following: position 201 of SEQ ID NO: 2 (SNP_02), or a position that is in linkage disequilibrium with this position,--position 201 of SEQ ID NO: 4 (SNP_04), or a position that is in linkage disequilibrium with this position, and position 201 of SEQ ID NO: 9 (SNPJ39), or a position that is in linkage disequilibrium with this position, and b) thereby determining whether the dog is susceptible to periodontitis.