On the X chromosome of a cat is a gene - often referred to as the O gene -which determines if the melanin pigment to be used is eumelanin or phaeomelanin. If the gene is switched on, phaeomelanin is used. If the gene is switched off, eumelanin is used.
If a cat has a dense pattern of phaeomelanin, it is a red cat. If it has a less dense or dilute pattern of phaeomelanin, it is a cream cat. Whether there is a dense or dilute pattern of melanin is determined by another gene called the dilute gene.
This confusion causes some parts of the cat to be red coloured, and other parts to be non-red coloured.
This is why nearly all torties are females. However some males can be torties due to having two X chromosomes. These males are nearly always infertile.
When naming the colour of a cat with this pattern, we give the name of the colour caused by the eumelanin and then add Tortie after that. For example Chocolate Tortie means the eumelanin colour caused by one X chromosome is chocolate, and the other X chromosome is creating phaeomelanin.
The pattern on each tortie is unpredictable and always unique.