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.
Actually Jeremy, the gray/white thing is easy. “White” horses are born white and stay white. As you noted, a gray is born dark and as more and more white hairs replace the dark ones, lightens with age until they are completely white. Gray horses also have black skin (other than under any white markings that were present at birth) and dark eyes, regardless of the degree of lightening of their hair coat.
True white horses are not only born white, but also have pink skin and usually have blue eyes. (One form of the sabino gene sometimes results in a predominantly pink-skinned white horse with dark eyes). On a side note, there are no “albino” (pink skin, red eyes) horses. I’m not a geneticist and the stuff on white coloring makes my eyes cross, but the oversimplified version is that albino is a lethal in horses.
With a gray, you practically have a different-colored animal each year! They’re born dark, get a little bit gray, then more and more each year until white, but the thing I am waiting to hear about is an explanation of why some gray horses become “flea-bitten.” After they become completely white in hair coat, some go on in subsequent years to develop little red specks of hair, sort of like freckles, that begin to dot their white coat. Some grays only get a few,like slight flaws in marble, others get so many that they almost look like rust paint was spattered all over them (hence the “flea-bitten” term).
Hope that helps. We really aren’t trying to be superior, we just look at the whole lifespan!