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| by Rachel Pagones WHAT has the Equine Genome Project revealed about the ‘tail-male’ line in pedigrees – that is, a horse’s pedigree traced exclusively through its parental line of descent? Just that the Y chromosome, the one determining maleness, is the only thing passed uniquely through sires. “When you think about the tail-male line,” says Dr Matthew Binns, “all that represents is the Y chromosome.” So what does that tell us about tail-male inheritance? “It doesn’t,” is the answer. Binns adds: “Much as I hate to say it, the Y’s a tiny chromosome, the smallest chromosome, and in mammals it’s only got about 70 genes on it. So when you think there are 26,000 genes distributed over 30-odd chromosomes, and each chromosome has on average about 1,000 genes, Y is pretty weedy.” In fact, Y is a veritable desert when it comes to genetic variation. Many years ago, Binns set out to disprove the famous ‘three founding sires’ theory. He was determined to prove, through variation in the Y chromosome, that more than three stallions – the Byerley Turk, Darley Arabian and Godolphin Arabian – founded the breed. To his great surprise, and equal amusement, he found that not to be the case. “I wanted to show there were five or six Y chromosomes circulating,” he says. “Was I ever wrong – there was only one that we could distinguish. Not only could we not find any variation in thoroughbreds – so we couldn’t distinguish between the Y chromosomes of any of the three families – but neither could we distinguish between the Y chromosome of the Connemara horse, the Icelandic horse, the thoroughbred, the Arab, the Exmoor, you name it. “We looked at four males from 15 different breeds and it looks as if possibly only male offspring from one founding stallion was domesticated. So if you go back to humans domesticating stallions, that probably tells you that early man didn’t find it too easy to catch or tame male horses.” Intriguingly, females show far more variation. A uniquely female type of DNA, called mitochondrial DNA, is passed on just as reliably through females as the Y chromosome is through males. “It looks like there were several mitochondrial types, and it looks like mares were domesticated in three, four, maybe five different geographic regions around the world,” says Binns. “An interesting idea is that when man domesticated species, there were two ways things could happen: either you could transfer the progeny around or you could transfer the knowledge of how to do it. And it looks like with the mares the knowledge was transferred, but with the stallions, actually the progeny were domesticated and tamed.” |