DNA methylation controls the binding of proteins that control the 3-D structure of genes.

This is a lightly edited version of an article I wrote as a guest on Alison’s blog over a year ago, looking back a couple of years to show something of what epigenetics was bringing to genome biology. The science has advanced further again since, but I’m nicking it back onto my blog (with Alison’s permission!) as it sets up other articles I would like to write.

Human karyotype. (Source; Wikimedia Commons.)

My article followed one Alison wrote about epigenetics. I’d suggest you read that first, as it will help!

While I’ve simplified quite a bit of the science to make things a bit clearer, it is a lot to take in, but persevere and you might get a glimpse of some of what this epigenetics fuss it really all about. (Feel free to ask questions in the comments section.)

I wanted to introduce an aspect of epigenetics that interests me: specifying the use of genes through forming different chromatin loops. In the case I’m going to look at the structure of the gene depends on which parent the copy of the gene came from.

Humans are diploid: we have two copies of each chromosome, one from each parent, except in males there is usually only one X and one Y chromosome (but two of all the others). Ignoring the sex chromosomes in males, having two of each chromosome also means that we have two copies of each gene. Each of the two genes making up a pair of corresponding genes, one from each parent, is called an allele. The two alleles of a gene make up the genotype of that person for that gene.

For most genes, when the gene is needed, both alleles are expressed and roughly the same amount of the RNA each allele codes for is made. But in some cases, evolution has selected that one of the two alleles should be switched off.

Tortoise shell cats are an example of mosaic X-chromosome inactivation (Image source: Wikimedia Commons.)

Alison described one example of this in her article: dosage compensation in females “corrects” for having twice the number of X chromomome genes as needed by switching one copy off. Recapping on what she was saying, in the case of switching off the “extra” copy of the genes on the “second” X chromosome in females, the choice of if the copy from the father (paternal allele) or from the mother (maternal allele) is inactivated is random. The choice made is inherited in each cell line once that choice is made. Because there are many cells, each making a separate random choice of which allele to switch off, most female mammals are mosaics, with a mixture of cells with an active paternal X chromosome genes and with an active maternal X chromosome genes. (I believe, rodents and marsupials are exceptions to this rule.)

One example of this are tortoise shell cats. The choice of expressing the black or orange alleles for fur colour are randomly chosen over the cat’s body.

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