Reviewed September 10, 2026. Corrected the cloning chronology and removed explanations that inferred more from a coat pattern than the evidence allows.
A calico is a colour pattern, not a breed. The familiar combination is orange, dark pigment and white, and nearly all cats wearing it are female. The explanation involves a gene on the X chromosome and the way cells use that chromosome during development.
Two versions of an orange switch
A typical female cat has two X chromosomes and can carry an orange-associated variant on one but not the other. Different cell populations keep different X chromosomes active, producing a mosaic of orange and non-orange pigment. A typical XY male has only one X chromosome and therefore does not have that same combination. Stanford Medicine’s account of the orange-colour research explains how random X-inactivation contributes to tortoiseshell and calico coats.
This is a useful explanation of the pattern, not a chromosome test you can carry out by looking at a photograph. Rare exceptions exist, and coat colour alone cannot establish an individual cat’s chromosomes or fertility.
The molecular result published in 2025
Two research teams independently identified an X-linked deletion associated with orange fur, publishing in Current Biology in May 2025. The deletion affects the regulation of ARHGAP36, changing its activity in pigment cells. The Kyushu University report describes the genetic finding and the parallel Stanford study. It provides a molecular explanation for an inheritance pattern known long before scientists identified the underlying change.
There is no reason to read a personality profile into that result. Identifying a pigmentation mechanism does not establish the popular claim that orange cats are unusually chaotic or friendly.
Calico, tortoiseshell and white
In ordinary coat terminology, tortoiseshell describes a mixture of orange and dark colours, while calico includes distinct coloured patches with white. These descriptions occur across breeds. Diluted versions can show cream and grey-blue rather than the more saturated colours. A registry’s terminology may be more specific than everyday usage; the words describe appearance rather than a separate kind of cat.
Why male calicos need a more careful explanation
A 1975 chromosome review examined 25 male tortoiseshell or calico cats and found an XXY component in 16. It also reported other chromosome configurations, including mosaicism and chimerism; almost all cats in that series were sterile. This selected clinical series is evidence of several possible mechanisms. It is not a population survey that establishes a universal “one in 3,000” frequency.
An extra X chromosome can allow the relevant colour variants to coexist in a male, but it is not the only possible explanation. Claims about a particular animal require veterinary assessment, rather than a diagnosis from its coat.
What the cloned cat does—and does not—show
Texas A&M’s cloned cat CC was born on December 22, 2001. The university announced the result in February 2002, after confirming genetic identity. Our earlier article incorrectly placed the announcement in December.
CC and her donor Rainbow did not have identical coats. The university’s later account attributes differences in pattern and colour distribution to developmental factors. That supports a useful distinction between a genetic copy and an identical appearance. It does not, by itself, prove the particular “fresh X-inactivation lottery” mechanism our previous version asserted.
A pattern with a public identity
Maryland made the calico its state cat effective October 1, 2001. The state’s official account notes its shared colours with the Baltimore oriole and Baltimore checkerspot butterfly. Biology explains why the pattern is unusual; people supply the symbolism.
The next calico you meet is therefore a useful reminder that genes and development work together. You can appreciate the mosaic without treating the size of a patch as an exact clock of embryonic events, or its colour as a prediction of personality.
