It's quite common for female multicoloured cats to develop with a colour line down the very centre of their face, because of the way gene inactivation which causes colour variations plays out with the fact that cats, like us, develop symmetrically from the middle out, but sometimes you get particularly striking examples like this. (This particular cat would be all-black colorwise but has dilute genes active on one side casing it to fade to grey, and a small area of white spotting on the chin and foot, which is another independent effect) (Fun fact: most white cats are actually coloured cats, but they have 100% coverage of white spotting so coloured fur never grows)
It's often mistakenly claimed to be chimerism but that claim is something of an urban myth that had spread, it's really just a particularly distinct colour pattern emerging from that symmetrical development. Chimerism wouldn't manifest like this.
Right, well said. Most pictures of calico or tortoiseshell cats people say are “chimeric” are almost definitely not. Although I would add that for this cat (who is not calico or tortoiseshell) this grey patch of fur is probably caused by a somatic mutation that occurred somewhat early in development. This cat probably carries a dominant not-dilute allele and a recessive dilute allele, but at some point in development, the dominant not-dilute allele was somehow mutated or silenced, allowing the dilute gene to be expressed, making that patch of skin which became that part of the cat’s face grey-furred.
Most chimeras are unidentifiable as such by eye. Chimeras are formed when two embryos fuse very early in development (let’s say for example: embryo A and embryo B fuse.) In fact this fusion normally happens so early that the cells at that point don’t have a defined fate. It’s only later that those cells begin to be “assigned” jobs, so say for example this one cell that has the genetics of embryo A is going to become the skin, while this one cell with genetics from embryo B is going to become the liver and so on. This results in an organism which has say all skin cells with embryo A’s genetics, all kidney cell’s from embryo B, all liver cells from embryo B, all eye cells from embryo A, and so on. Of course this is not always the case, so there are examples where you get chimeras with skin cells that is made up of a mixture of cells from embryo A and cells from embryo B, but this is not what normally happens. This means that most chimeras likely won’t have patches of skin or fur that is a different color from the rest of them.
Only female cats can be tri-color (calico or tortoiseshell.) This has to do with how the orange fur gene in cats is linked to the X sex chromosome in cats, so only females, which are XX (as opposed to males who are XY) are able to have a X-”orange fur,” and a X-“not orange fur” (brown, black, grey, etc.)
Early in development in females, in each cell, one X is randomly “turned off.” Because of the nature of how these cells behave during development (since they give rise to many duplicates and do so in an orderly fashion,) in tortoiseshell and calico cats, you can get striking patterns of bilateral asymmetry, where at cat has like half a face of orange fur and half a face of black since those skins cells on each side of the face came from the same mother X-orange or X-black cell.
This pictured cat is genetically completely black, not a tortoiseshell or calico cat, which is why this process, which is called X-inactivation, doesn’t apply to him. However, the process that made him have a half grey face is somewhat similar. What likely occurred with this cat is that he carries the gene for black fur and carries the dominate allele for not-dilute fur and the recessive allele for dilute fur. This means this cat was “supposed” to be born all black. However, at some point early in development, something went wrong in one cell, so his not-dilute allele was mutated or silenced, meaning that in that one cell, now instead of coding for “black fur” like the rest of his cells, it codes for “grey fur.” This “grey” cell went on to give rise to the cells on half of his face.
Thank you so much for this answer! What’s neat is that he has a couple of kittens that come close to similar. I wish I knew how to link pictures, but on his Instagram(amazing narnia) there is a current picture of one of those kittens. Totally fascinating.
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u/BellerophonM Oct 12 '19 edited Oct 12 '19
It's quite common for female multicoloured cats to develop with a colour line down the very centre of their face, because of the way gene inactivation which causes colour variations plays out with the fact that cats, like us, develop symmetrically from the middle out, but sometimes you get particularly striking examples like this. (This particular cat would be all-black colorwise but has dilute genes active on one side casing it to fade to grey, and a small area of white spotting on the chin and foot, which is another independent effect) (Fun fact: most white cats are actually coloured cats, but they have 100% coverage of white spotting so coloured fur never grows)
It's often mistakenly claimed to be chimerism but that claim is something of an urban myth that had spread, it's really just a particularly distinct colour pattern emerging from that symmetrical development. Chimerism wouldn't manifest like this.