X-chromosome inactivation, mosaics – or, chimeras?

X-chromosome inactivation, mosaics – or, chimeras?

I’m running Schol Bio tuts again this year, & I like to start each session off with a question on some aspect of biology, just to start everyone thinking in that space. In the last session I presented two images (see below) and asked the students:

  • what are possible genetic explanations for what you’re seeing here, and
  • what feature of inheritance is NOT applicable here?

Venus, the “two-faced” cat Photo: Venusmommy

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A domestic half-sider budgerigar. Photo: Susan Dennis – https://www.flickr.com/photos/23506271@N06/26690893991

The first response to my questions was, are we looking at either incomplete dominance or codominance? My answer was that it’s not; in fact, it’s the feature of inheritance that is definitely not in play.

Why not? Well, because with both of those, the alleles involved are expressed at the cellular level. An example of incomplete dominance is the pink flower morph in snapdragons. Red flowers have 2 copies of the allele coding for red pigment ; white flowered plants are homozygous for white; and pink-flowered plants have one copy of ‘red’ and one of ‘white’, which is expressed as pink. But the flower is pink throughout, because both alleles are expressed in each cell in the petals. And the AB blood group in humans are a good example of codominance, where both alleles are expressed simultaneously (see also MN blood types). Here, someone with AB blood expresses both A and B markers on their red blood cells.

So, if not codominance or incomplete dominance, then what are we seeing in the cat & the budgie? There are a couple of possible options.

As several students pointed out, with Venus (the cat) we are quite probably looking at the results of a phenomenon known as X-chromosome inactivation. This is something that’s seen in all placental¹ mammals with a pair of X chromosomes. Very early in embryonic develepment – around the 1000-cell stage – one of those X chromosomes is randomly inactivated in every one of those cells. The chromosome is tightly coiled (heterochromatised), such that only a few genes on one end can be expressed. Functionally, the result of this is that males and females have the same dose of X chromosome genes & the proteins for which they code. And XX individuals are mosaics for inactivated and activated X chromosomes. Because brown/black and orange coat colour in cats is encoded in a gene locus on the X chromosomes, the result of X-chromosome inactivation is a cat with patches of orange and black fur, randomly scattered through its coat, giving us one explanation for Venus’s sharply-divided bicolour face.

A second explanation is that Venus is a chimera: an individual animal that’s formed from two or more zygotes, so that there are different genotypes present in different parts of their body². This is often not visible to surface inspection – the budgie being a striking exception – but can turn up in DNA work for parenthood cases. The Lydia Fairchild case is an absolutely fascinating example of this.

And this is the explanation for the half-sider budgie: it’s what’s known as a tetragametic chimera, which is formed when 2 fertilised embyros fuse very early in development – before the 64-cell stage. The two halves of the resulting embryo are genetically distinct, although this will only be visible if the two originating embryos would have resulted in birds of different colours. Usually half-siders appear divided down the long axis of the body, but apparently if they are formed from a later-stage fusion they can look much “splotchier”.

We had such an interesting discussion 🙂

 

 

¹ However, in marsupials it’s always the paternal X chromosome (the one inherited from the father) that’s inactivated. (It’s not clear what’s going on in monotremes.) I did not know this before doing a bit of reading for this post.

² In class we also talked a little about microchimerism, where during pregnancy a woman picks up very small amounts of fetal tissue (& DNA) that become incorporated into her tissues; the reverse can happen as well.

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