As you might well imagine, the reason why tortoiseshell cats are nearly always female is to do with the genetics and how the genes that dictate the coat colour and pattern are linked to the sex of the cat. Female mammals have 2 X chromosomes while males have one X and one Y chromosome.
The particular genes which control the particular colour form of the tortoiseshell cat are attached to or carried by the 2 X chromosomes only (both chromosomes are needed). The gene that produces red colour is on one X chromosome and the non-red gene is on the other.
RELATED: I have commented more on the above cat and the dilution of colour on another page [link].
In other words, the genes which create the tortoiseshell coat are carried on X chromosomes and it requires 2 X chromosomes to carry the genes which produce the tortoiseshell coat colour. This means that only females are tortoiseshell cats except for the rare exceptions. To put it another way there is a physical attachment between the genes that create the tortoiseshell coat and the chromosomes which dictate whether a cat is female or male.
Dr. Desmond Morris states that the chances of finding a male tortoiseshell cat have been calculated at about 200 to 1. Other Internet sources state that the chances of finding a male tortoiseshell cat are at 3000 to 1. I prefer the latter statistic.
Male cats, as you know, have one X chromosome and one small Y chromosome. On their single X chromosome, they carry either the red or the non-red gene, but they can’t have both. Therefore, their offspring are either all-over red tabbies or all-over black.
Male tortoiseshells should not exist but occasionally there is a minor genetic error, and a male tortoiseshell cat comes into existence, developing with the genetic combination XXY. The 2 X chromosomes in the genetic makeup allows the cat to be red and black while the Y chromosome makes the cat male. Male torties are called ‘the unicorn of cats’. You can read more on this topic by clicking on the link below.
Dr. Morris states that male tortoiseshell cats are “masculinized females”. They have peculiar personalities he thinks. They avoid the usual male battles because they don’t feel the need to compete in these “single-sex pecking-order disputes”. And one male tortoiseshell cat was observed to not attempt to mate with females on heat even though they were anatomically equipped to do so.
Note: The exact same genetics and rare occurrence of male cats occur in calico cats (tortoiseshell-and-white). And of course, the dilute version of the tortie is also affected likewise.
Below are some more articles on tortoiseshell cats.