S P E C I A L REPORT
IT IS WIDELY understood that the ancestry of the Thoroughbred dates back over 300 years to three foundation stallions – the Darley Arabian, the Godolphin Arabian and the Byerly Turk (seen below) – hailing from North Africa and the Middle East.
During the 17th and 18th centuries these stallions were bred to select mares, native to Britain, and so began the process of selective breeding.
In 1791 the first English-based “stud book” was compiled by James Weatherby.
Through his own research and by consolidation of his privately kept pedigree records the foremost volume of the General Stud Book was published listing 387 mares, each of which could be traced back to Eclipse – a direct descendant of the three founding sires of the Thoroughbred breed.
By 1873 the American Stud Book was in print. Founded by Colonel Sanders D. Bruce who relentlessly researched and collated the pedigrees of American Thoroughbreds, it was shortly thereafter taken over by The Jockey Club under whose jurisdiction it still remains today. As in the case of the General Stud Book, the extended pedigrees of any racehorse in North America disclose an irrefutable link to the original founding sires and dams, in particular Eclipse (90%) who was a dominant undefeated racehorse and phenomenal success as a sire.
Fast forward to the 20th century.
By mid-century the North American fervor for refinement of the breed had begun its fateful course of speed at the expense of soundness. A paradigm shift in the deep-rooted culture of the racing world occurred which would interminably transform the “Sport of Kings”.
Prior to this development the majority of the preeminent stallions and mares were controlled by the richest sport patrons belonging to some of the oldest established families in the US – the Whitneys and Woodwards, the Bradleys and Wideners and the Klebergs and Mellons. A fundamental rule that these families persevered to abide by was the diligent effort to improve the breed from a multi-attribute perspective; speed, stamina and soundness.
In the 1950’s the gradual dissolution of these families together with the ensuing auction sales of their unparalleled bloodstock changed the once stately nature of horse racing. Commercialization of the industry now loomed - an ominous portent of impending doom to the integrity of the Thoroughbred.
Motivated by the prospect of increased profits the commercial breeders laid a foundation for what would become the archetype of the modern Thoroughbred. Rather than durability and resilience they opted to fashion an aesthetically pleasing horse with pedigrees replete with celebrated status, in particular those sires with unsurpassed speed thus drawing the biggest price at the most extravagant yearling sales. Designer “genes” became the sought-after prototype.
Reminiscent of the eugenics movement during the Hitler regime the development of perilously inbred pedigrees fatefully arose. The influx of vulnerable gene pools began predominantly with the immortal Native Dancer, nicknamed the “Gray Ghost” because of his color. Native Dancer’s brilliance includes a record 21 victories in 22 starts, tarnished only by a loss to Dark Star in the 1953 Kentucky Derby. However accomplished his racing career was, it was nonetheless short-lived.
"By the time Native Dancer had reached age 4, when he started only three times through August, he had gotten so sore due to a chronic inflammation in his ankles -- he reportedly had developed osselets, bony growths along his ankle joints -- that his owner and breeder, Alfred G. Vanderbilt, was forced to retire him to Sagamore, Vanderbilt's Maryland farm." 
It was here at Vanderbilt’s Sagamore farm that Native Dancer went on to even greater renown as a stud, emerging as one of the most influential sires in the history of the breed. In particular, his grandson, the Canadian born Northern Dancer, was the founding sire of the most fashionable and prolific sire line in the world. After Northern Dancer’s death another Native Dancer grandson called Mr. Prospector - extraordinarily fast but unsound - moved to the top of the commercial market to become the next superstar sire who would continue to infuse the bloodlines with speedy but compromised genes in terms of soundness.
Multiple generations of calculated selective breeding has resulted in a very narrow gene pool wherein Native Dancer’s bloodline has been estimated to be found in about 75% of all US thoroughbreds. In fact, nearing the end of the 1960’s the pursuit of ultimate speed and mounting competition infiltrated the global horse racing industry. Through the use of shuttle stallions major racing jurisdictions in Europe and the UK as well as the Southern Hemisphere (i.e. South America, Australia and New Zealand) started to cross-breed their indigenous long-standing bloodlines with Native Dancer’s pedigree, most notably Northern Dancer and Raise a Native, grandson and son respectively.
As a result of commercialization, market forces and greed the entire global Thoroughbred population is now so inundated with the blood of Native Dancer that any counterbalances that would thwart the passage of these vulnerable genes has virtually been absorbed leading to an escalation in the amount of inbreeding currently present in the racing world. As the gene pool shrinks it brings with it a most undesirable trait.
"Like hemophilia in the Russian royal family, Native Dancer's line has a tragic flaw. Thanks in part to heavily muscled legs and a violent, herky-jerky running style, Native Dancer and his descendants have had trouble with their feet. Injuries have cut short the careers of several of his most famous kin, most notably Barbaro, a great-great-great-grandson who was injured during the Preakness Stakes and was later put to death." 
Despite 135 years of championship breeding, just two bloodlines dominate the sport at the present time. The blood of Native Dancer and Seattle Slew, out of the British Nasrullah line, courses through the veins of nearly every prominent race horse running today.
Part 1: Breeding for Breakdowns | Part 2: The Rise of the Ill-Fated Gene Pool | Part 3: Commercialization: The Descent of the Thoroughbred | Part 4: On the Brink of Extinction?