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Göte, I knew there was a down side to that. Spot with sex linked Mark. Black (B) and ginger (G) alleles (forms of a single gene) are on the X chromosome. Females are XX and males XY. XBXB = black female; XGXG = ginger female; XBXG = tortoiseshell female; XBY = black Tom; XGY = ginger Tom. The odd tortoiseshell Tom is XBXGY due to a mutation in which the cat has XXY (Klinefelter's syndrome in humans - you, given to parents in a blue blanket but the extra X expresses itself at puberty).