Nearly every male Thoroughbred descends from one Arabian stallion imported in 1704
Every modern Thoroughbred traces back to three stallions shipped to England between the 1680s and 1729. One genetic study found that about 95 percent of male Thoroughbreds carry the Y chromosome of just one of them, the Darley Arabian, whose line survived through an unbeaten racehorse named Eclipse.
Breeders in England shaped it across the 1600s and 1700s by mating local mares with stallions of Arabian, Barb and Turkoman stock. The founding trio were the Byerley Turk, the Darley Arabian and the Godolphin Arabian, though some 160 stallions contributed in all. Each founder's male line hangs on a single later descendant: Matchem for the Godolphin, Herod for the Byerley Turk and Eclipse for the Darley. Across all lines, though, the Godolphin Arabian actually appears more often in modern pedigrees than the Darley.
Racing is far older than the breed. Four-mile races were run at Smithfield in London by 1174, and handicapping, adding weight to even out chances, arose under James I. Royal enthusiasm under Charles II and Anne made racing popular, a dedicated Racing Calendar appeared by 1727, and the word thro-bred was first used in 1713. The General Stud Book followed in 1791. Once the Classic races such as the St Leger of 1776 and the Derby of 1780 were set at one to 1.75 miles, breeders shifted from long, multi-heat contests to faster, younger horses.
Selection worked. In 1885 an ordinary Thoroughbred, Iambic, carrying 29 kilograms more than the best Arabian of the day, beat him by 20 lengths over three miles. Fear of American horses flooding British racing produced the Jersey Act of 1913, which barred horses without fully registered ancestry from the stud book until 1949. The first Thoroughbred in the American colonies, Bulle Rock, arrived in 1730.
Thoroughbreds average about 16 hands and are usually bay, brown, chestnut, black or grey. Around 100,000 foals are registered each year, and every Northern Hemisphere horse officially ages a year on 1 January. The demands of racing bring problems, including high accident rates, bleeding from the lungs, small hearts and low fertility, and their causes are still being researched.
Source: Thoroughbred
