This page explains the genetics behind the fur pattern on the face of Venus, the famous “two-faced” calico cat. There is no doubt that Venus is a calico cat by which I mean a tortoiseshell-and-white cat. She has some white fur on her chest.
By “explain” I mean to the best of my ability and within the limits of the current knowledge of cat genetics. When you read Robinson’s Genetics for Cat Breeders & Veterinarians you realize that there is still quite a bit to know.
Below is a collage that provides some initial information and below that I write further about the genetics behind Venus, the two-faced cat.
What I would argue is that Venus is actually a regular calico cat. “Calico”, incidentally, is an American term for the tortoiseshell and white cat.
The genes that produce the orange, black and white fur of the calico cat’s coat are listed in the collage above.
The gene that has produced the stark line that demarcates the boundary between the black fur (eumelanin) and the orange fur (phaeomelanin) is the white spotting or piebald gene.
The piebald gene is the one that creates the particolor faces of Ragdolls. These are pointed cats with a sharp white pattern on the points. Also the Snowshoe cat shows this sharp line produced by the white spotting gene (see picture).
In the case of Venus, the white spotting has caused an initial lack of color on the right side of Venus’s face (as we look at her) by stopping the migration of pigment producing cells to that side of her face. That would have left that side white. However, the presence of the orange fur gene (the O gene) has lead to the creation of cells that produce pigment for orange fur which have “stepped into the shoes” of the cells that do not produce pigment.
The presence of the white spotting gene has affected the color of Venus’s iris. The eye on the right as we look at her is blue. This is an iris without pigmentation. The white spotting gene is known to cause odd-eye color and a blue eye.
The fact that the line is exactly down the middle of the face is a one in a million chance, which makes Venus special. The pictures of the calico cat and black and white cat in the collage above show that the sharp demarcation on the face is commonplace. It is just a question of where the line (blaze) is and between which colors.
It is very hard to control the effect of the white spotting gene in breeding Snowshoe and Ragdoll cats. Venus is not a purebred cat. Ragdolls and Snowshoes are purebred and breeders use selective breeding to create the well positioned sharp lines between white and black.
For Venus, a random bred cat, nature took its course and by pure chance created a beautiful and arresting cat for humans. Cats themselves of course have no aesthetic preferences in respect of coat color and pattern.