The Genetics of Venus the Famous Cat

Her name is Venus. She's a fast-rising video and social media star, noted for her strikingly unusual coloration. Half her face is black and half is orange and the two colors are split down the middle. And her eyes are different colors.

Higgs (an orange tabby) has been quite intrigued. He says before he was neutered he would have enjoyed conducting some genetic experiments with her, but now that he lives a life of the mind, he'd just like to know how such an interesting look comes about.

Some people have speculated that Venus is a chimera – a term that refers to an animal with two different kinds of cells each with a distinct genetic code. Scientists can create chimeric mice and monkeys in the laboratory by mixing cells from two genetically distinct embryos.

Occasionally two would-be fraternal twins meld together and the result is a human chimara. As long as the twins were going to be the same sex, there may be nothing unusual-looking about such a person.

Venus, however, is probably no more of a chimera than any other female mammal, according to geneticist Greg Barsh of Stanford U and HudsonAlpha Institute for Biotechnology. What you're seeing, he said, is a particularly striking manifestation of a phenomenon called X inactivation. Female mammals have two different kinds of cells because we have two X chromosomes and yet only one is active in any given cell. The other one is permanently shut down. Which X is activated varies around the body. If you're a woman, you have patches of cells with your father's X active, and other patches of cells with your mother's X active.

In cats, X inactivation can lead to interesting visual effects, since some of them carry a mutation on the X chromosome that leads to orange fur. Any male inheriting this mutation is orange, since he has just one X chromosome. To be all orange, a female would have to inherit the mutation from her father and her mother. If a black or grey cat says he's the father of an orange female kitten, he's mistaken.

Things get interesting if a female cat gets one X with the orange mutation and one X without it. She will end up with patches of orange and patches of some other color – usually black or brown – and in some cases white too, which qualifies a cat as calico. Barsh says this is probably the explanation for Venus's coloration. Eyes are even more complicated to explain, but he doesn't believe that different eye colors necessarily indicate that she's a chimera.

Venus' black color comes from a gene on another chromosome, Barsh said. In one form that gene leads to black, and in another, to a brown color known as agouti. Venus looks to have two copies of the black version and on her X chromosomes, one copy of the orange mutation. What makes her unusual is that all the cells on the orange side of her face carry the X with the orange mutation turned on, and all the cells on the black side of her face have the X with the orange mutation turned off.

Higgs wants to know what would happen if he had mated with her before he was neutered. Here's my guess: Since he's orange, their female kittens could be orange or patchy (tortoise shell) and their male kittens could be orange or black.

Barsh says that's close. It really depends on whether Higgs carries black or brown (aka agouti) genes, which are on another chromosome and invisible since his orange mutation automatically makes him orange. If Higgs carries agouti genes, his male cats could be brown tabbies and his female offspring might have brown patches instead of black ones.

Some readers may have seen a similar cat on the December page of the 2012 calendar from the Philadelphia Animal Welfare Society(PAWS). That cat, "two face" appeared to have agouti fur on one half of her face rather than black, and she had an astonishing tail. Though she is every bit as beautiful as Venus, she was unwanted, reportedly in poor condition and malnourished when she was turned into a PAWS shelter by her owner. She had been in a shelter for 50 days when the calendar went to press.