An easy way to mark yourself as a novice is to call a white horse white. They’re greys. I don’t know why, but there it is. ((Probably specifically to enable horsey types to act superior.)) Today, thanks to a paper in Nature Genetics, I do know why they’re grey. ((Gerli Rosengren Pielberg, Anna Golovko, Elisabeth Sundström, Ino Curik, Johan Lennartsson, Monika H Seltenhammer, Thomas Druml, Matthew Binns, Carolyn Fitzsimmons, Gabriella Lindgren, Kaj Sandberg, Roswitha Baumung, Monika Vetterlein, Sara Strömberg, Manfred Grabherr, Claire Wade, Kerstin Lindblad-Toh, Fredrik Pontén, Carl-Henrik Heldin, Johann Sölkner, Leif Andersson (2008). A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse Nature Genetics, (), – DOI: 10.1038/ng.185)) Turns out they are over-expressing two genes on horse chromosome 25, thanks to a duplicated bit of DNA 4600 base pairs long. And that’s true in more than 800 greys from 8 different breeds. the duplication has not been found in any non-grey horses.
Greys generally start off with dark hair, but gradually lose the dark pigment, leaving them with white hair and, usually, black-skin. Alas, greys also often develop melanomas that reduce their lifespan, and also show depigmentation of the skin like viteligo in humans. So what’s going on? Leif Andersson and his colleagues suggest that maybe the two genes, called STX17 and NR4A3, may be speeding up the rate of division of pigment cells in the skin and the hair follicles. In the skin, this leads to melanomas. In the hair follicles it depletes the stem cells.
