And now the racing results ... 1st: Nurture, 2nd: Nature

It is the lifeblood upon which the sport of kings depends: the millions of pounds spent each year on the sperm of race-winning horses. The owner of one superstar stud can earn £25m a year.

But for breeders, and stallions looking forward to an active retirement, scientists have some bad news: a horse's lineage is far less important than was previously thought. Research published today will cast doubt on the rationale that bringing champion horses together will produce potentially race-winning foals. Nurture, the study says, trumps nature when it comes to dictating which horses win.

Alistair Wilson, an evolutionary biologist at the University of Edinburgh, found that genes accounted for only 10% of the prize money a horse wins in its lifetime, with the remainder determined by how a horse is reared, trained and ridden.

British breeders can typically expect to pay more than £500 a time in stud fees, with some American horses commanding fees of up to £10,000. Stallions reputed for producing good quality offspring come at a premium, and can fetch far higher fees. But the report determined stud fees are "not an honest signal" of a stallion's genetic quality.

Wilson analysed the lifetime earnings of more than 4,000 horses used for racing and breeding since 1922. Because the money a horse earns in its life is determined by genes to a minor extent, selective breeding will have some influence over a horse's abilities, he found.

"But while there are good genes to be bought, it does not appear that you get what you pay for," Wilson says today in the journal Biology Letters.

"The offspring of expensive stallions might tend to win more money, but not necessarily because they have inherited the best genes," said Wilson. "It is likely those breeders best able to pay high stud fees are also those who are able to spend more on care of the horse, how it is trained, and who rides it - all of which will contribute more to how much it will win."

He said breeders should not assume they will get winning foals by simply paying the highest stud fees. "They would be better off worrying less about studs and concentrating on what happens afterwards. Or, if you're in a position where you're trying to buy a stallion, if you can work out which are the best males, genetically speaking, you can get yourself a bargain because the best genes don't necessarily come with the highest price tag."

There are situations where good genes become important: if every breeder spent a similar amount on the care of a horse through its life, for example, then the difference between a winning or losing horse would come down to the details, such as the parents. In that situation, picking good parents could give a foal an edge.

Twink Allen, head of the Equine Fertility Unit in Newmarket and father-in-law of jockey Frankie Dettori, said that the results were not surprising. "The paper is saying that the price of a stallion doesn't relate to the winnability of its progeny - that's true. A stallion acquires a price on what the market thinks of it, like a work of art or something like that, but whether it's up to that takes time to tell."

After a stallion retires from racing, it is given a value based on its pedigree and how successful it was on the track. Allen said a stallion might begin its stud career by covering mares at a charge of £20,000 each and stay at that level for a few years, simply because it takes that long before its foals get on to a racecourse. "Then the stallions either go up or down depending on the ability of their progeny. A lot go down steadily and slip out the back door. Some, which seem to get a higher-than-average rate of winners, go roaring up."

"The whole thing about racehorse breeding is that it is speculative by nature," said Neil Willmott, owner of the Brookfield Stud in Norfolk. He said that the parentage of a horse was important but warned: "Just because it's expensive, doesn't mean it's going to win. You only have to look at the sales at Ascot to look at the number of horses that come out of training and people have spent a lot of money buying the colt or filly as a yearling and then they suddenly find it hasn't made the grade and they're selling it on very cheaply."