XX/XY chimerism in tortoiseshell tomcats – a new case and review of the literature

Introduction: Tortoiseshell coat color in cats typically occurs in females due to random X-chromosome inactivation, which affects the expression of the orange coat color gene. The presence of a tortoiseshell phenotype in male cats usually indicates an unusual sex chromosome constitution. Case presentation: A young Maine Coon tomcat with a normally developed penis was referred for genetic examination because of its tortoiseshell coat. At the age of 11 months, he successfully mated with a queen, which subsequently gave birth to five healthy kittens. Results: Cytogenetic analysis of chromosome preparations obtained from leukocyte cultures revealed the presence of two cell lines - XX and XY. Molecular studies were performed on DNA extracted from blood, hair follicles, and saliva using a ddPCR approach to estimate the ratio of Y- to X-chromosome copy numbers. These analyses confirmed the presence of both XX and XY cell lines in the tested tissues, with the highest proportion of the XX line detected in hair follicles. Conclusion: This study demonstrates that tortoiseshell tomcats with XX/XY chimerism can be fertile. Therefore, cytogenetic and molecular analyses of tri-colored male cats are recommended before making decisions regarding their use in breeding.