You know why virtually all tortoiseshell cats are female, right? It’s all due to genetics.  Well, maybe some of you don’t, so a quick explanation is in order to introduce today’s felids.

Tortoiseshell cats, or “torties,” are cats with patches of orange and black fur, and sometimes a bit of white. Here’s one:

How do they differ from calico cats? Only because calicos have more white in their fur.  But, like torties, calicos are almost always female.  Here’s a calico:

Why are these cats nearly always female? It’s because, as in humans, female cats have two similar sex chromosomes (XX), while males are XY, with the Y chromosome largely genetically inert (it does carry sex-determining genes).  It turns out that the X chromosome (but not the depauperate Y) carries a gene that affects fur color, and that gene has two forms, a “black” form and an “orange” form.

The other thing you need to know is that, during development of females, one X chromosome is completely inactivated in each cell. That has evolved to keep the “dosage” of gene products identical between the sexes. So an XY male produces product from one X chromosome, and so do XX females, because one X is switched off in each cell (this mechanism is called “dosage compensation”).

The inactivation occurs at different stages of development, and is random, so, for example, early in development one cat embryo cell will have one of its X’s inactivated, and all the descendants of that cell will have the same X inactivated.  But since inactivation doesn’t occur at the very beginning, a live female cat will be a mosaic, with one of the X’s inactivated in part of its body and the other X in other parts. Which X is inactivated seems to be random.

So imagine a female cat with two X chromosomes, one carrying the gene coding for black fur, and the other for orange.  If the black-gene-carrying X is inactivated, those epidermal cells will express only the orange gene, producing orange fur. If the “orange” X is inactivated, the cat makes black fur. If a female carries both forms of the gene (i.e. she’s a “heterozygote” for black-X and red-X), the random inactivation of X’s will lead to a cat having patches of red and black fur, i.e., a tortie.  Males, having only one X, cannot be mosaics, and so will be either all black or all red. That is why male calicos and torties are almost nonexistent.

What about the calicos? Well, to be a calico you still have to have red and black patches, ergo you have to be female, but some of these cats have an additional gene, called “piebald”, which is not on the sex chromosomes. That gene has a form, S, causing white spotting (Hili has it), and is dominant over the alternative form “s”, so if you have two copies of the piebald form (S/S) you get white patches, if one copy (S/s) copies you have smaller white patches, and if you have no copies (s/s) you don’t have any white.  The only difference between calicos and torties, then, is that calicos have at least one S form of the gene as well as being heterozygous for the X-linked black and red genes.  Male cats, of course, can also show piebaldness, since the gene is not on a sex chromosome.

If you want this all explained with diagrams, go here, or go here for a longer explanation.

That is a long-winded but necessary introduction to a true rarity, a male tortoiseshell cat named Harry who has shown up at a cat shelter in Bonnyrigg, Scotland; his owners were allergic to his fur. Here’s the cutie, just a 3-month-old kitten:

Harry with Nicola Zelent from Lothian Cat rescue, photos from the Daily Record (link above):

How can you have a male tortoiseshell cat? It has to have two X chromosomes to get that fur pattern. But it also needs a Y chromosome to be a male. And, indeed, the rare male torties have the chromosome constitution XXY, and are heterozygous for the X-linked fur-color genes. But because their sex chromosome balance is messed up, these male cats are sterile; they are, in effect, “intersexes,” but I suspect are identified as males because they have male-like genitals.  To quote from the article:

Margaret Riddell [the vet] said: “When I heard the cat was called Harry, I said to the owners, ‘I think that might have to be a Harriet’.

“I had to change my words when I discovered it was male. I’ve never seen one before and I’ve been a vet for more than 30 years.”

Well, it’s not really a “male” in the conventional sense; it’s an intersex. But no matter, it’s rare, it’s cute, and the good news is that it doesn’t have to be neutered.

The XXY condition occurs in humans, too, though rarely (in roughly 1 in 1000 individuals identified as male). It produces what’s called Klinefelter Syndrome. Because of the unbalanced sex chromosomes, that condition, as in cats, produces sterile males as well as a variety of traits described by the NIH:

Affected individuals typically have small testes that do not produce as much testosterone as usual. Testosterone is the hormone that directs male sexual development before birth and during puberty. A shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement (gynecomastia), reduced facial and body hair, and an inability to have biological children (infertility). Some affected individuals also have genital differences including undescended testes (cryptorchidism), the opening of the urethra on the underside of the penis (hypospadias), or an unusually small penis (micropenis).

I suspect that an inspection of Harry’s willy (note the two royal names) would show abnormal but male-looking genitals.

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Okay, enough biology for the morning. Here’s a cat with paper-related OCD in a video that’s garnered 1.8 million views in the last nine days. Who knows what it’s up to?

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College Humor has a series of gifs showing cats giving high fives. Here are a few:

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Finally, as lagniappe, at the latest Oatmeal the cats get their back: