Interesting how different news outlets interpret a story. The Telegraph's High price may not make champion horse reports an interesting result of a study by Alastair Wilson and Andrew Rambaut at the University of Edinburgh finding that stud fees are a poor marker for genetic quality of racehorses. The Guardian concludes And now the racing results ... 1st: Nurture, 2nd: Nature. The Times focuses on the small genetic component as crucial: 10% factor that makes a champion. Go figure.
None of the coverage mentions a further source of confusion: that the breeders themselves are unlikely to select stock optimally, due to working by outdated folk theories of horse genetics. This Pedigree Dynamics article, Conception and Misconceptions - A light hearted look at breeding theories of the past, look at some of them, mentioning how theories such as telegony and "mental impression" survived in the thoroughbred horse breeding industry well into the 20th century.
Even now, racehorse breeding is governed by many semi-empirical racehorse breeding theories that don't bear much relation to real-world genetics. For instance, some breeders place store in the "X factor" (possession of a Large Heart gene). Others rate the horse's heritage according to its place in the Bruce Lowe Family Numbers classification. Yet others go by Dosage (closeness of relationship to chefs-de-race, the relatively rare consistently winning stallions) or "nicking patterns" (the belief, debunked here, that particular pairs of bloodlines may be statistically identified as producing winners when mated); or even rules-of-thumb that smack of outright numerology ("a mare’s third foal, if born when the mare is seven years old, is the most likely to be a successful racehorse").
Given such a muddle of beliefs, it's unsurprising that nurture should turn out to be the dominant factor in racehorse success.