I know red (and cream) are referred to as sex linked colours, but I think all colour is on the X chromosome.? As red and black are the only true colours (everything else, inc white, being due to different genes causing dilution, coat pattern etc) you can only get both if you have 2 X chromosomes - thus male torties being XXY male which are almost always sterile, although I think it is possible for them to be fertile. Without the 2 X chromosomes males can only be black or red, but of course females can be black and red and thus tortie.