This Friday I am teaching an introductory lecture on genetic switches, so I had a look around on the web for topical examples. My favourite is recent work by Guenther, Kingsley and others in Stanford on work on the genetics of blond hair. Of course we know what blond hair must be determined by our genes, blond parents tend to have blond children. But we are still working out what are the genes involved. Amongst our twenty-odd thousand genes is KITLG, which like a a lot of our genes is involved in a lot of different things, including how our bodies make blood, and whether we have blond hair.
There a couple of variants of this gene, and one appears to reduce the pigment in your hair, making it likely you will have some shade of blond hair. Hair colour is determined by a bunch of different genes, which is how we, both blondes and those with darker hair, have a whole range of hair colours.
Guenther et al. suggest that the mechanism is that blondes have a version of the KITLG gene that is less active, because the mutated DNA makes it harder for another protein, called LEF-1 to switch it on. This is well described in a blog post here. The cells of our bodies are run by a fearsomely complex network of genes, in which thousands of genes regulate (i.e., switch on and off) other genes. My brown hair is probably a consequence of one of these connections being rather stronger than it is in (natural) blondes.