Meet my cats, Echo and Luna:
Figure 1: Meet Luna and Echo
Perhaps not the greatest wild- (or domestic-) life photo ever taken, but it will do for today's purposes, and at least they are both looking at the camera.
You will observe that Luna is a black cat. Echo, on the other hand, seems to be what you would get if you randomly mixed a black and a brown cat together and did not stir very thoroughly.
When we first took them to the vet, she checked Luna first and declared her to be female. She then declared Echo to be female on account of her being a tortoiseshell - or "tortie" - but double-checked to confirm her initial assumption.
So what is going on here?
- Why can we be almost certain that a tortie is female?
- Why are torties coloured as they are?
- And why can we never be one hundred percent certain that a tortie is female?
To answer these questions we need to delve into some cat genetics.
The figure below shows a schematic representation of a cat cell.
Figure 2: Schematic cat karyotype (NB chromosomes do not really have this shape or these colours and they are not all the same size)
The cat-cell nucleus contains thirty-eight chromosomes, but chromosomes usually come in pairs and there are eighteen pairs of chromosomes - thirty-six in all - that are shown in blue and labelled A1, A1; A2, A2 etc. These chromosomes are known as autosomes.
There is also one pair of sex chromosomes shown in yellow. These are the ones we are really interested in here (though the autosomes will get another mention).
Figure 3: Schematic cat cell nucleus showing only the sex chromosomes
One of these is always an X chromosome. If the other is also an X, you end up with a female cat. If the other sex chromosome is a Y, you end up with a male cat.
Echo is female and has two X chromosomes; and the two genes (or, more precisely, the two alleles of the same gene) that make her black and brown lie on her two X chromosomes. They are, in the jargon, X-linked.
Figure 4: Tortoiseshell cat X chromosomes
The point here is that the gene in question cannot occur on the Y chromosome. So an XY, male cat would have one X chromosome with either a black or a brown gene and would thus be either completely black or completely brown.
Of course, male and female cats can be all sorts of other colours and have different sorts of patterning, but that involves various other genes. And "brown" may be more of a cinnamon, orange, or ginger hue. And the colour scheme as a whole can be well defined or muted. Here we are only considering the genes that produce the tortoiseshell appearance, and trying to keep things simple.
Thus we have already answered the first question we posed. If a cat has a tortoiseshell coat, we know that it has two X chromosomes and we can, therefore, be almost certain that it is a female.
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But what about our second question. Why the strange random patterning?
In order to appreciate just how strange this is, we might consider what would happen if the black and brown alleles were not X-linked but occurred on one of the autosomal pairs - say the A1 pair.
Figure 5: Hypothetical cat with autosomal genes for black coat colour
If a hypothetical cat had two copies of the gene for black colouring on its A1 chromosomes, it would (unsurprisingly) turn out black (see above).
Figure 6: Hypothetical cat with autosomal genes for brown coat colour
If, on the other hand, a hypothetical cat had two copies of the gene for brown colouring on its A1 chromosomes, it would (unsurprisingly) turn out brown (see above).
Figure 7: Hypothetical cat with autosomal genes for black and brown coat colour
And finally (and less unsurprisingly) if a hypothetical cat had one copy of the gene for black colouring and one copy of the gene for brown colouring on its A1 chromosomes, it would nevertheless turn out completely brown (see above).
I suppose we could, a priori, imagine that such a hypothetical cat might turn out an even shade of very dark brown. Sometime genes can work a bit like that and produce a kind of blend of their effects. But that is not what would happen here. The brown allele of the gene would be dominant over the black allele and the result would be a brown cat.
Back to real cats with X-linked coat colour genes and we get a completely different type of result:
Figure 8: Real cat with X-linked genes for black and brown coat colour
We see random alternating patches of black and brown fur.
The reason for this strange result is something called lyonization. Lyonization is the process whereby one of the two X chromosomes in every cell of a female individual is inactivated (in a randomly chosen fashion) during development of the female embryo. One explanation for lyonization is that it prevents females from making twice as much of whatever the genes on its X-chromosomes are coding for as a male (with only one X chromosome) would make - though, like many things in biology, it is a bit more complicated than that.
Lyonization is named after the British geneticist who discovered it: Mary Lyon. She, like all other females - human or otherwise - also had half her X chromosomes randomly turned off or on, but because humans do not have genes for skin colour on their X-chromosomes, this fact was[i] not so obvious as it is in the case of my feline family member.
We do sometimes see similar things in humans however. The general term for the phenomenon whereby an individual is composed of two or more genetically distinct[ii] cell lines that are descendants of a single fertilized egg is called mosaicism. Random mutations occur every time a cell divides, so, even if we are male and do not have lyonized X-chromosomes, we could all be said to be mosaics. But this term is normally applied where the genetically different cell lines produce significant or visible effects – as they may do (for example) when people end up with different coloured eyes: heterochromia.
When Echo was still a developing embryo, some of the cells in that embryo had just had their black-allele-X-chromosomes turned off and some just had their brown-allele-X-chromosomes turned off and some were waiting to be randomly switched one way or the other. As these cells divided and proliferated, whatever way they were switched was passed down to all their descendants - though all were switched one way or the other before long.
Each of these two cell lines randomly ended up in what became Echo's fur producing cells and she thereby acquired her random patches of black and brown fur.
And this answers the second question posed above. Echo really is rather like a random mixture of two genetically distinct cats.
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So how could a tortie possibly (in rare cases) turn out to be male?
Well one possibility is something called Klinefelter's syndrome where males are born with an extra X chromosome. Such individuals are XXY instead of the conventional XY. Because lyonization switches off one X chromosome in each cell, it might be imagined that there are no differences between males with Klinefelter's and males with more conventional karyotypes but, as has been noted, nothing is simple in biology. Klinefelter's tends to cause developmental problems and infertility.
A male cat with Klinefelter's and a gene for black fur on one X chromosome and a gene for brown fur on the other X chromosome will end up with tortoiseshell colouring just like the conventional female tortie. Such a case has been described by a team at the University of Copenhagen [iii].
Another possibility is something called chimerism. Whereas mosaicism occurs when two different cell lines emerge within a single embryo, chimerism occurs when two different embryos merge to form a single embryo. The end result of either mosaicism or chimerism may be a single embryo containing different cell lines but they are very different phenomena.
If we imagine that twin embryos originally destined to become a black tom and a ginger tom merge during embryonic development, the eventual outcome could be a single male kitten with tortoiseshell colouring. I have no idea whether such a cat has ever been discovered and investigated, but it is certainly a possibility.
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When I last took Echo and Luna for their jabs in late October, the vet had a young student observing her. "Oh a black cat and an orange and black cat! Just perfect for Halloween!" exclaimed the student.
At Halloween (or Samhain to use its original name) the veil between the surface world and the underworld thins. I hope I have succeeded in thinning the veil between what happens on the surface of a cat and what is going on underneath.
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