Calico Genetics

BY Wendy Christensen

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What Makes A Calico, A Calico?

"Oh, you have a money cat!" exclaimed my neighbor upon meeting my calico, Petunia. "You could sell her, for a lot!" Petunia was worth the world to me, but I'd never heard of anybody cashing in on a shelter kitten. My neighbor was predictably vague about just who might be willing to write me that big check. But my curiosity had been piqued. How had the apparently common belief that tri-colored cats were worth big bucks gotten started? Was it an urban myth? Or was there more to it? And what makes a calico, a calico, anyway?

One thing's certain -- owners of calicos know their cats are special. They brag about sparkling personalities and striking looks, often attributing both to what some call the "fire gene." "Most veterinarians will attest that in general, calicos and torties have a more 'mercurial' or 'intense' personality," agrees Jerold S. Bell, Clinical Assistant Professor of Genetics at Tufts University School of Veterinary Medicine in North Grafton, MA. But there's no single "calico gene." Calico and tortoiseshell cats result from a complex chain of interactions very early in a kitten's development.

Each of a cat's 38 chromosomes is part of a nicely-matched pair --except those dubbed "X" and "Y," the sex chromosomes. Normally, each kitten gets an X from Momcat and either an X or Y from Dad. When a male cat's sperm fertilizes a female's egg, It bequeaths either an X or Y to the embryonic kitten, determining its gender. A normal female cat has two X's ("XX"); a normal male cat has one X and one Y ("XY").

The Y chromosome, the one that determines maleness, is much shorter, and carries less genetic information, than the X. Therefore, the X pulls a lot more weight in determining certain physical traits. The gene for orange coat color (called "O") is carried only on the X chromosome, so it's called a sex-linked gene. "O" specifies either orange fur or not-orange (i.e., black) fur. Interestingly, while "O" is the only sex-linked color-coat gene in cats, in animals other than cats that can display "calico" coats, the relevant genes are not sex-linked!

The female's two X's obviously carry some redundant genetic information, so nature has devised a clever and splendidly efficient way of sorting out which genes will determine the kitten's phenotype, or physical makeup. This process is called random X-inactivation. In concept, the process is marvelously simple: One X chromosome in each of the developing embryo's cells is "turned off." (At that point, that X chromosome is "compressed" into a mere shadow of its former self and is thereafter called a Barr body.) X-inactivation is random, irreversible, and is passed down to all the cell's descendents. So henceforth, the developing kitten will be composed of cells descended from two different cell lines: the ones in which the maternal X was inactivated, and the ones in which the paternal X was turned off. In other words, she's a mosaic.

What does this have to do with calicos? Remember, "O" is carried on X. If both maternal and paternal X's said "this kitten will have orange in her coat," she will. And if both X's said, "nope, no orange fur on this kitten!" she won’t have any orange hair. So in many cases, there's no conflict, at least about orange coloration -- the two cell lines agree. The interesting case occurs when one X says "orange" and the other X says "no orange -- just black!"

Because the kitten's X chromosomes were inactivated randomly, she now has two distinct cell lines: one that's set to produce orange fur and one that's coded for black. This is reflected in her coat color and pattern, with areas of orange and black fur determined by the random inactivation of her X's. If she also carries the piebald, or "white spotting" gene, she'll be a calico, and her red and black fur will appear in solid, distinct patches. If she inherited the piebald gene from both parents, she’ll have more white fur than if she inherited it from only one. And the more white fur she has, the clearer and more distinct her black and orange patches will be.

If she lacks the piebald gene, she'll be a tortoiseshell, with her orange and black fur randomly swirled, woven and intermingled throughout her coat in a tweedy mix, rather than occurring in distinct patches. And if she inherited the "dilution" gene, she'll be a blue-cream cat, either a "dilute calico" or "dilute tortie," depending on whether she has the piebald gene.

Between the varying timing and the randomness of the x-inactivation process; other genes that can influence the coat; and certain environmental conditions within the womb that can also affect the size and placement of coat colors and patterns, it’s virtually guaranteed that no two calicos or tortoiseshells look alike. And whatever their patterns, calico beauty is at least skin-deep. Michelle West of Tourmaline Scottish Folds in Toronto, Canada, had to shave her calico Fold, Punkin, because of severe matting caused by illness. She was delighted to discover that "Punkin's skin was black where she had black fur, but her white and red parts were all pink -- the red parts being a slightly darker pink."

Male Calicos, and the Birth of a Legend

Since a normal male cat has only one X chromosome, the "redundant X" conflict never comes up. His one X chromosome simply decides that either he's going to have orange fur, or he isn’t. End of story, right? Most of the time, yes. But occasionally, in very early embryonic development, a kitten can acquire, though genetic errors, one or more "extra" chromosomes. This can result in a wide range of unusual genetic combinations, also called mosaics. One of the commonest manifestations of this kind of genetic anomaly is called Klinefelter syndrome, in which a normal XY male acquires an extra X. This can occur in many mammals, including humans, dogs, pigs, mice -- and cats.

In XXY (Klinefelter) males (and other males with multiple X's), the same conflict -- two X's with possibly conflicting codes -- that arises in normal females must be dealt with. Efficient nature uses the same process: random x-inactivation. So, if one of the XXY male's X's says "orange fur, please!" and the other says "no way -- just black!" the result is a calico or tortoiseshell. But because of other abnormalities caused by the anomalous extra X, a Klinefelter male, of any species, is invariably sterile.

So from whence comes the rare, but reliable, reports of fertile male calicos? Some of these result from unusual forms of mosaicism, in which a fortuitous distribution of cells may render the cat either partially or fully fertile. Others, much rarer, result from a process called chimerism. Chimerism is a special case of mosaicism that occurs when two or more embryos fuse together to become a single individual -- a chimera. If, very early in development, a brother and sister embryo (an XX and an XY) fuse and successfully become a single kitten, that kitten will be a "mosaic" of XX (female) cells and XY (male) cells -- an XX/XY chimera. If his sperm cells are XY, he can father kittens just as a normal male can.

There's one thing that's clear about male calicos -- they're rare! Even many experts and veterinarians have never met one. "I have not seen any male calicos in my practice," notes Bell. Of the nine veterinarians with practices in different parts of the US I surveyed, four had never seen a male calico, and four had seen only one (and one of those was in a university's breeding colony). One veterinarian, though, had seen "several," and had even owned two of her own! "In the literature," notes Bell, "29 percent of male calicos are Klinefelter's (XXY). 18.5 percent were chimeric (XX/XY) individuals, 16 percent were fertile XY male cats, and the remainder were combinations of mosaic and chimeric individuals."

And what about that "money cat" myth? It seems to have been born of a curious braid of three strands -- an ancient Asian cultural tradition, some English and Irish folklore, and hazy memories of mid-twentieth-century genetics research.

Maneki Neko, the famous Asian "beckoning cat" with upraised paw who has long been considered a symbol of prosperity and bringer of wealth, is usually portrayed as a "mi-ke" (three-fur") Japanese bobtail, mostly white with widely-separated patches of black and orange. It's said that tortoiseshell and calico ships' cats were highly prized by Japanese fishermen, who credited these unique felines not only with protecting them from storms, but even with shielding the crew from the jealous spirits of their ancestors.

Old English and Irish folktales also identify "penny cats" or "money cats" -- calicos -- as lucky charms. A sure-fire cure for warts called for rubbing the wart with the tail of a tortoiseshell tomcat -- but only during the month of May! The moniker "money cat" probably crossed the ocean with immigrants, and has since taken up residence in the vernacular of many regions of the US.

And it was likely the modern scientific search for the genetic roots of the anomalous male calicos (and the even rarer fertile male calicos) that gave new life to such tales. It’s said that genetic researchers, perhaps at a US university in the 1950s or 1960s, put out a call -- and offered a small bounty -- for male calicos so that their genotypes could be studied.

Though their monetary value may not be all it's sometimes cracked up to be, calicos are irresistibly appealing, often requested and among the first adopted from shelters. So who says they aren't truly "lucky cats?" If you're the proud owner of a calico -- even a fertile male calico! -- don’t count on him to finance your retirement. Treasure your calico's splendid uniqueness, and glory in the fact that she -- or he! -- is truly one of a kind.


  • Brindled: Not patched; intermingled hairs of different colors.
  • Calico: Properly, "tortoiseshell with white;" a white cat with unbrindled (solid) patches of orange and black.
  • Chimera: An individual embryo formed from the fusion, very early in development, of two separately-fertilized eggs. Some male calicos result from chimerism.
  • Chromosomes: Threadlike strands of DNA in the nucleus of each of a cat's cells that carry genes. A cat has 38 chromosomes, in 19 pairs. Half are inherited from mother, half from father.
  • Dilute: a gene that dilutes, or softens, orange to peachy-cream, and black to a soft silver-grey, often called blue.
  • Gene: An individual sequence of DNA or RNA that occupies a particular position on a chromosome, and carries a protein "code" that controls the transmission and expression of one or more traits such as coat color or eye color.
  • Mosaic: An individual composed of different cell lines. Some male calico cats result from mosaicism.
  • Torbie: A brown patched tabby, combining brown tabby and red tabby markings. Torbies are often mistakenly called calicos.
  • Tortoiseshell: A cat with brindled (intermingled) patches of red and black (or blue and cream, if dilute) hair.
About The Author: Assisted by her family of feline muses, Wendy Christensen writes and paints at Bobcat Mountain Studios on a wooded hillside in rural New Hampshire . Wendy is a multi-award-winning writer, the author of scores of articles and books on feline topics. She’s also an award-winning illustrator and artist specializing in cat portraiture. Her paintings have appeared in several books, magazines and on numerous giftware and other products. As a cat behavior consultant, Wendy helps puzzled cat owners sort through their options in solving feline behavior dilemmas, especially in multi-cat settings. (Need help with a feline dilemma? Email her.) She's a member of the Cat Writers' Association (CWA); The Dog Writers Association of America (DWAA); and the Guild of Natural Science Illustrators.



  • Jerold S Bell, DVM
    Clinical Assistant Professor of Genetics
    Department of Clinical Sciences
    Tufts University School of Veterinary Medicine
    fax 860-749-4760
  • Michelle West
    640 Roselawn Ave., #218
    Toronto Ontario M5N 1K9
  • "Robinson's Genetic for Cat Breeders and Veterinarians" (Fourth edition, 1999) by Vella, Shelton, McGonagle and Stanglein
  • "Cats are not Peas: A Calico History of Genetics" by Laura Gould (1996, Springer-Verlag)

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