Thoroughbred Blood Percentage is the genetic percentage shared with a reference population of known Thoroughbreds (Petersen, et al), referred to in Eventing circles as, "% Blood." Etalon's ancestry testing measures this percentage genetically, and due to higher accuracy, may differ from pedigree calculations by as much as 30%.
Pedigrees provide an approximation of genomic data, calculated based on the idea that every time you breed horses the sire passes on 50% of his genetics and dam passes on 50% of her genetics. However, we know that this isn’t always the same exact 50%, because of instances called “recombination events” that occur within reproductive cells, oftentimes two full siblings may have very different Ancestry percentages. This is true for humans as well -- chances are you didn’t get exactly the same 50% of your DNA from your mom and 50% from your dad as your siblings, but a balance somewhere in the middle.
Genomic measure is much more accurate. We can measure genomic data from a horse’s DNA and compare that Ancestry to the Thoroughbred population. Meanwhile, a horse’s pedigree is just a proxy. Before we could measure this value from actual genetics, we relied on pedigrees to make crucial breeding decisions. Now, however, with genetic testing available, pedigree-based analysis of inbreeding and ancestry is quickly becoming “outdated.”
Let’s take a look at Tsetserleg. Based on his pedigree, he should have 47% Thoroughbred. After DNA testing, we see that he has much more - he is actually 62% Thoroughbred. This difference is worth noting, as Thoroughbred Blood Percentage has a real effect on a horse’s performance at various disciplines, especially on the cross-country field.