At the Better With Pets conference that I attended last year, someone in the audience asked a panel expert a question about her cat, a “tortie,” a cat with tortoiseshell markings of orange and black patches. The expert said, “What’s her name?” And the person asked how they knew that her tortie cat was female. “Because all torties are female,” the expert replied.
The means of the genes
And it’s true! Well, almost. Without diving into a biology lesson, the gene that determines orange and black fur on a tortie is found only on the X chromosome but it needs to be paired in a cell with another X chromosome with the alternate gene for orange and black for a tortie to occur (so one gene in the cell has to be orange and the other gene has to be black, or more accurately, “not-orange” which results in the expression of black and darker colours). So right away, it stands to reason that only females can become torties because they have two X chromosomes (XX), while male cats, like male humans, have just one X and one Y chromosome (XY). If this is the gene that will determine the male’s colour — other genes are at work to determine other colours and patterns — the male will be either orange or black because they only have one X chromosome, and therefore a single gene is expressed throughout the body as orange or black.
Well, we’ve already dipped our toes in the biology lesson, so let’s keep going. What happens in females if they have the orange gene on one X chromosome and its “not-orange” counterpart gene on the other X chromosome is that the orange and black genes will duke it out to make all the individual cells display either orange or black fur in a random mash-up all over the body in a process called X-inactivation. This happens because you can’t have a cell with two active genes, and the female body equalizes things by rendering one of the pair of X chromosomes inactive, ensuring that both males and females have the same amount of expressed X-linked genes. Are you with me? Therefore, the orange parts on a tortie result from the black (not-orange) genes being inactivated in the cells whereas the black patches result from the orange genes being inactivated in the cells. Again, it’s a completely random process what cells will display which colour. Other genes determine patterns, markings, dilutions, and other colours, but two X chromosomes must always be present for a tortie to occur.
Phew! We got there. Even if you can’t wrap your head around the whole process, at the base of it all, a tortie must have two X chromosomes. So you would think that makes all torties female. Not quite.
Two X chromosomes makes all torties female, then, right?
In extremely rare cases, a male can display tortie markings either because of a chromosomal disorder called Klinefelter syndrome, which results in two X chromosomes, XXY, and whose primary feature is sterility, or because two fertilized eggs fused to make one cat, a chimerism. The chances of either happening is so rare, most veterinarians never come across them. One such cat born in Scotland in 2014 took his vet by surprise who said, “I’ve never seen one before and I’ve been a vet for more than 30 years.” Another one from England said, “[a]s a vet I can tell you that it is genetically impossible to get a male cat that is tortoiseshell coloured.”
How impossible? The England article puts their odds at 400,000 to 1 while the Scottish paper says, “[t]here are around 8 million pet cats in the UK and only a couple each year are born male tortoiseshell.” Some articles like this one and on Wikipedia say the chances are 1 in 3,000, but with references to outdated links and old books, I can’t verify (if you can find hard data, let me know). For a more in-depth explanation of the science, you can start with the Wikipedia article and its resources at the bottom of the page.
So far on this blog, we’ve met Turtle (pictured above) who is Tribble’s sister, and Martha from the Rescue Report, both female torties. Do you have a male tortoiseshell cat or have you met any male torties? I’d love to hear about it!