O Locus
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Sex-linked red. Responsible for tortoiseshell (tortie), tortoiseshell tabby (torbie), calico, and calico tabby (caliby). The red allele O only exists on the X chromosome, because the Y chromosome is significantly degenerated and has less space for genes on it, for that reason the tortie pattern is distinct to females except in extenuating circumstances. The O locus determines if eumelanin (black and brown pigment) is allowed to be produced by any given cell or if pheomelanin (red and orange pigment) is produced instead. Sex-linked red is epistatic to eumelanin and will always express as tabby, even if it is genetically solid/self. Both pigments are co-dominant and when present together, X-inactivation takes place causing every cell to deactivate one copy of the X chromosome and resulting in patches of orange and black pigment as seen in torties or calicos.
Causes of male tortie: Klinefelter mosaic males (XXY), somatic mutation (from cells passed between embryo during development), and chimera mosaic male (from two embryos fusing either XX/XY or XY/XY). Klinefelter mosaic males and chimeric mosaic males (with the XX/XY) combination are always sterile, while somatic mutations are not genetic and will not be passed, XY/XY chimeras will be fertile and may breed true for only one set of chromosomes, or they may be able to pass either (what is called a "true breeding tortie male").
- XOXO or XOY = red based (pheomelanin)
XoXo or XoY = black based (eumelanin)
XOXo = tortoiseshell
XXY = Klinefelter mosaic male
XX/XY = Sterile chimera mosaic male
XY/XY = Fertile chimera mosaic male
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