Linebreeding in the Kentucky Derby: Part Two

A continuation of Monday's post examining inbreeding trends among Kentucky Derby winners

On Monday, I began delving into the history of linebreeding in the Kentucky Derby, beginning with the first running of the race in 1875 and going through 1949. Today, I will be covering the 1950 through 2024 runnings of the Kentucky Derby, and discussing some potential takeaways.

The 1950s (1950-1959)

• Average COI: 3.09%

• Highest COI: Tomy Lee (5.77%)

• Lowest COI: Needles (1.26%)

• Influential Ancestors: Stockwell continued to reign as the most frequently repeated ancestor, appearing 15+ times in the pedigree of every Derby winner this decade except for Needles

There was quite a bit of close inbreeding among Kentucky Derby winners this decade, with four Kentucky Derby winners having a repeated ancestor in the third generation. Tomy Lee was inbred 3S x 3D to Hyperion and 4S x 4D to Swynford; Tim Tam was inbred 3S x 3D to Bull Dog, 5S x 5D x 5D to Spearmint, and 5D x 5D to the mare Cerito; Iron Liege had the full siblings Bull Dog and Sir Gallahad 2S x 3D; and Determine was inbred 3S x 4D to Gainsborough.

Both Hill Gail and Needles were free of inbreeding within five generations, and both featured Galopin as their most-concentrated ancestor, comprising 7.81% blood influence in Hill Gail’s pedigree and 9.96% influence in Needles’ pedigree. Dark Star’s sire Royal Gem carried two copies of Son-In-Law, but he was not inbred across his pedigree within five generations.

The great mare Iron Maiden was the dam of 1954 Kentucky Derby winner Iron Liege and the second dam of 1952 Kentucky Derby winner Swaps. Bull Dog also featured prominently in multiple Derby winners this decade - his son Bull Lea sired both Hill Gail and Iron Liege, and his daughter Isolde produced Dark Star.