If the legendary breeder Federico Tesio was correct in his assertion that gray color in Thoroughbreds is a form of skin disease, the Alcock Arabian could be considered patient zero.
Every modern gray Thoroughbred traces directly back to the stallion through an unbroken line of like-colored ancestors, weaving through gray sires and dams across centuries to arrive at any gray racehorse on the end of a shank. For example, Tapit, the record-setting sire with a striking near-white coat, carries 30 generations of gray ancestors from the original source.
Though the Alcock Arabian's sire line fizzled after a handful of generations, Tesio found his influence on the breed to be so great, he labeled him the fourth foundation Thoroughbred sire, joining better-known male-line cornerstones the Darley Arabian, Godolphin Barb, and Byerley Turk. Where individual traits of the three best-known foundation sires have assimilated into the general Thoroughbred gene pool, the Alcock Arabian's direct impact can be seen at any racetrack or auction.
How does one horse from the 1700s leave such an indelible mark on the breed without a notable succession of sons? The reasons vary from on-track performance and breeder preference to the power of genetics.
Like many notable horses of the early 18th century, the origins of the Alcock Arabian are murky. By some accounts, he was imported to England from Constantinople – modern-day Turkey – in 1704. However, the noted English breeder and pedigree researcher Lady Wentworth found evidence to suggest he was born in England. Just as confounding was the variety of names the horse went by, owing to changes of ownership, legend, and unreliable record keeping. Lady Wentworth linked him to a litany of other aliases, the best known of them being “Mr. Pelham's Grey Arabian.”
What history can generally can agree upon is that the stallion eventually ended up in the hands of a man known only in the record books as “Mr. Alcock,” who owned a stud farm in Lincolnshire, England, and gave the horse the basis for his name.
While there were plenty of other gray horses entering the gene pool in England during the early 18th century, the Alcock Arabian's legacy certainly benefitted from being the first stallion recognized for his gray coloring in the British General Stud Book in the early 1700s.
His genetic influence spread early, earning the leading sire title by earnings in Great Britain and Ireland in 1728 on the strength of his best son, Crab (fitting for the time, he was also known as “Old Crab” and “Mr. Panton's Crab”). That horse went on to become a sire of influence in the region, being named Great Britain and Ireland's leading sire each year from 1748 to 1750.
It remained a viable sire line through the rest of the 1700s, culminating with Aimwell, winner of the 1785 Epsom Derby, making him the only winner of England's derby to descend from a male line other than the three foundation sires. However, Aimwell's generation was unable to produce a meaningful son at stud, and the Alcock Arabian's presence on the very top of pedigrees stalled out.
Fortunately, the handful of generations that preceded the end of the Alcock Arabian's male line produced enough gray mares to preserve the color in the breed, if at a slower pace; bottlenecking at the broodmare Bab – born in 1787, six generations removed from the originator – and spreading back out through her 10 gray foals.
Dr. D. Phillip Sponenberg, a professor of pathology and genetics at the Virginia-Maryland College of Veterinary Medicine, said that early success by the sons and daughters of the Alcock Arabian, and the next few generations that followed, likely secured the future of gray Thoroughbreds as they are known today.
“I would have expected that the original Thoroughbreds varied more for color than now, and originally there may well have been other grays besides Alcock,” he said. “I suspect in Alcock's situation that the production of a son as a popular sire would have broadly reproduced his gray color. Had the line been uniformly nonproductive, then the gray could easily have become extinct. That said, some [other gray] lines probably fizzled because they were slower or otherwise less productive.
“But, at some point, the English also started shunning certain colors in horses, and that might also have played a role,” Sponenberg continued. “I do suspect that they might have had more color variation originally, and that things like dun, palomino, and 'true roans' simply died out early.”
The direct-line preservation of gray coloring from the Alcock Arabian to modern Thoroughbreds, Sponenberg explained, is an example of a dominant color gene asserting itself.
Because gray color is a dominant gene, he said, all gray horses have the potential to pass their color down to their offspring at least 50 percent of the time. The dominant gene ensures that gray color does not skip generations, meaning either the sire or dam of a gray Thoroughbred must be gray themselves.
While the origin of the Alcock Arabian is open for debate, his last days are documented about as reliably as the stud book of the day would allow. In 1722, the stallion was sold by Mr. Alcock to the Peregrine Bertie, the second Duke of Ancaster, and he died the following year having sired just five recorded foals for his new owner.
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