For Racing's Blue Bloods, It's Thin Blood

The only race Native Dancer ever lost was the 1953 Kentucky Derby, but in industry circles he is best known as the most valuable sire ever. All 20 horses in this year's Derby are descendants of Native Dancer, as is about 75% of the thoroughbred population. But with those champion genes come some drawbacks, writes the Wall Street Journal.

All of the inbreeding meant to produce better racers has also let a genetic weakness shine through: foot trouble of the kind that felled Barbaro (a distant grandson). Driven by economics, the thoroughbred gene pool has shallowed: Though births are up, the number of stallion fathers has been cut in half since 1992. "How much can we keep breeding into these same bloodlines?" said one expert. "We're dealing with the law of diminishing returns." (More horse racing stories.)