more on mosaics

A couple of days ago I posted a stunning photo of a peacock, and talked a bit about the possible genetic underpinnings of its colour patterns. My friend & blog-buddy Grant then pointed me at the story of a cat that has a similarly amazing colouration. Venus even has her own Facebook page! (I will definitely be using both these images & their stories with my first-year class next week.)

As you can see,  one half of Venus’s face is black, with a yellow eye, while the other half is orange – with a blue eye. What a stunning cat!

The underlying reason for her striking colouration isn’t clear. She could be a chimera, formed from the fusion of two fertilised eggs, so that some of her cell lines would have different DNA from the rest. She could equally well be a particularly impressive example of the results of X-chromosome inactivation in female mammals. In that case, of the two X chromosomes in the cells of the right-hand-side of Venus’s face, the one with the ‘orange’ allele would have been inactivated. That side of her face is black because all cells have functioning X chromosomes that are expressing the ‘black’ allele. The reverse would be true for the orange side of her face.

Which leaves the question: why does she have a blue eye on the orange side?

In the comments thread to my original post, herr doktor bimler noted that

[the] X-chromosome mosaicism of female mammals turned up a lot in colour-vision research, because women carrying a colour-deficiency gene on one X chromosome end up with their retinas being more-or-less coarse patchworks of colour blindness. In the extreme case people can be red-green deficient in one eye and normal in the other**.

This can also be true for haemophilia, where the gene locus involved in the most common form of this disease is found on the X chromosome. (Haemophilia C is not sex-linked.) Heterozygous women, with one normal and one recessive, deleterious allele, are regarded as carriers, unaffected but with the potential to pass the ‘haemophilia’ allele on to some of their children, with damaging effect in those sons who receive it. But – and I must get my class to consider this one next week – such women are mosaics for X chromosome genes. If the ‘haemophilia’ X chromosome is ‘on’ in all the cells of their bone marrow, those women would also be haemophiliac.


**Being a person with a dry sense of humour, herr doktor concluded that such women "must spend their lives hiding from the colour vision researchers who wish to experiment upon them."

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