The tortoiseshell coat colour is characteristic to female cats, and its 
occurrence in tomcats is very rare and associated with chromosome abnormalities 
(additional copy of X chromosome). The aim of this study was identification of 
the genetic basis of a case of tortoiseshell colour in a fertile Maine coon 
tomcat. Cytogenetic and molecular genetic studies were carried out with painting 
molecular probes (WCPP) specific to the X and Y sex chromosomes as well as a DNA 
microsatellite panel for the parentage verification of cats. Cytogenetic 
analysis revealed only a single set of sex chromosomes typical for male - 38,XY. 
The results of the microsatellite polymorphism obtained from DNA showed three 
alleles in locus FCA201 and four alleles in loci FCA149 and FCA441 in different 
tissues (blood, hair roots and testicles). Based on these results, the case was 
diagnosed as a true chimerism 38,XY/38,XY. To the best of our knowledge, this is 
the first case of a 38,XY/38,XY chimera diagnosed in cats, confirmed by genetic 
analysis.