Inbreeding in the Thoroughbred

Rock Sand

Rock Sand 1900, English Thoroughbred

A new study has revealed, that contrary to the current industry understanding, that inbreeding in the Thoroughbred has experienced a consistent decrease since 1900, resulting in the modern Thoroughbred being 18.7% less inbred than its ancestors!

Let me introduce you to Sean Kilroy, of Clonmel, Tipperary, Ireland. Sean, a racing enthusiast since the 1960s, racehorse breeder, and has been evaluating pedigrees since he discovered the work of Nick Mordin in 1992, who analyzed the pedigrees of that year's English Derby field and produced a short list of four that included the winner and the second. Like Mordin, and most of us pedigree researchers of that era, Sean had to find his pedigrees in books and periodicals, and then write them out by hand, as online databases and pedigree programs were still in the future.  He has studied dosage, and the various evaluation systems of racing ability, plus, he read and contacted early greats like Jack Glengarry, Simon Morris-inventor of TesioPower, and accessed the work of Harold Hampton, Clive Harper etc. He built up his own database on his original TesioPower, and went on to write several of his own pedigree programs so he could calculate the hard numbers (he is an accountant) of inbreeding, and he has kindly shared the fruits of this research with us. 

In this newly completed study of the inbreeding of 618,275 Thoroughbreds, from 1900 to 2019, Sean evaluated each horse for ten generations in two ways, and most important for us breeders are the inbreeding truths he found demonstrated in this genetic history. His methods, and his findings are summarized in the following short article, which he provided for us on short notice. I advise you to print it out and read it through a few times. Each sentence is loaded with documented information. You may contact Sean for an Excel spreadsheet showing the progress he used to arrive at the conclusions at

´╗┐Inbreeding in Thoroughbreds by Sean Kilroy