Extra reading on tortoiseshell cats
What makes some areas in a tortoiseshell red and others black?A few days after conception, when the original cell has divided itself in many similar cells, one of the x-chromosomes is shut down in each cell. For the rest of the life there is only one x-chromosome in the cell that is functioning. Randomly it's decided which x-chromosome is to shut down. When the cells again are dividing themselves the daughter cells will have the same x-chromosome shut downed as the mother cell. This will make the cat have a mix of red and black markings.
A male can't be tortoiseshell since he only has one x-chromosome. Still, sometimes a tortoiseshell male appears in a litter. How can this be? Must he then not have two x-chromosomes? Yes, this is exactly how some of the events of the tortoiseshell males are explained. Some of them have something wrong with their chromosomes, they have two x-chromosomes and one y-chromosome. Either the mother put two x-chromosomes in the egg, or the father gave both his x-chromosome and his y-chromosome to the sperm. The same chromosomal aberration occurs in many other animals and in humans. It is called Klinefelter's syndrome. This chromosome aberration leads to sterility - when a male tortoiseshell is not sterile, then he is tortoiseshell because of another reason.
__________________ | | | | | | X | Y | |____|_____|_____| | | | | | XX | XXX | XXY | |____|_____|_____| | | | | | 0* | X0 | Y0 | |____|_____|_____| *) No sex-linked chromosome
XXX - meta-female, to humans: look normal, at least some of them, maybe all, are fertile.
The tortie coloring of a male may also be caused by a fault in the pigmentation of the skin. This is often seen in a red cat, that has some spots, sometimes even large spots, of black. These males breed as a red male. In principle, one might get a black tomcat with a bit of red. Such a cat however has never been reported.
Yet another variety of tortoiseshell males may be caused when two fertilized eggs, maybe already grown to a group of cells, are united and becomes one embryo. This is the reverse of one fertilized egg splitting into two embryos, that become identical twins. If one of the fertilized eggs would have become a black tomcat, while the other would have become a red tomcat, then the combined embryo will be a tortie male cat. There has for example been born a red-blue and white tabby Maine Coon in the US, who has been shown to be the result of such a combination of fertilized cells. The male is fertile and has given only the red color to his kittens.
Of course, these anomalies can also occur where both X-chromosomes carry the same color. Then the result is not a tortie tom, but a plain normal red (cream) or black (or variant) male cat. These males will never be suspected of being something special.
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