Inbreeding Notations in Thoroughbred Pedigrees

In a recent survey about Thoroughbred breeding and pedigree content on, I asked readers to suggest topics they'd like to read about more often. A frequently-repeated request was to run "introductory" articles about various breeding theories.  Apparently it can be confusing to have text referring to "5 x 3 inbreeding" or "sire line affinities" or "a Brilliant/Intermediate Dosage profile" without explaining what is meant.  I'm all for it -- and I'll start out with an occasional Back to Basics post.  If there's a specific topic that you think requires a short tutorial, please indicate in the comments section and I'll put it on the list!

Let's start with inbreeding.

Right away, we need to clear up some definitions. The Thoroughbred breeder uses slightly different terminology than other livestock breeders.  Inbreeding refers specifically to a duplication of a single ancestor within five generations.  A concept that is related but distinct is line-breeding, which refers to duplication of a single ancestor within more than five generations.  (Note that this definition differs significantly from how "linebreeding" is used in other livestock breeding.)

Inbreeding and line-breeding both use generational notations.  If Teddy appears in the fourth generation of a horse's sire and also the fifth generation of its dam, that horse is said to be "inbred 4 x 5 to Teddy."  See an example here.

The generations are always denoted according to the duplicated ancestor's appearance on the pedigree while regarding it vertically.  (Huh?!?)  Here's an example: start looking at Ikigai's pedigree from the top down. You'll see one cross of Mr. Prospector in the fourth generation (as sire of Gone West) ... another in the fifth generation (as sire of Fappiano) ... and another in the fourth generation (as sire of Conquistador Cielo).  Therefore, Ikigai is inbred 4 x 5 x 4 to Mr. Prospector.  (He's also inbred 4 x 5 to Reviewer and 5 x 5 to Arts and Letters.)  (And as an aside, it's pretty fitting that Ikigai won the grade III Mr. Prospector Stakes!) 

Some sources take it even further and indicate whether the duplications come through the sire or the dam, using "S" and "D" distinctions.  This is useful when there are three or more duplications of the inbred ancestor. In the case of Ikigai, for example, he has Mr. Prospector 4S x 5S x 4D.  Taking it one step further, some publications will use case to indicate whether the duplication came through sons or daughters.  Again looking at Ikigai, we'd see that his inbreeding to Arts and Letters is 5s x 5D.  Note the lower case "s" means that the first instance of Arts and Letters came through Ikigai's sire (the letter "s") and specifically through one of his daughters (small case instead of capital letter). The upper case "D" means that the second instance of Arts and Letters came through Ikigai's dam (the letter "D") and specifically through one of his sons (capital instead of lower case).

Here's where things get a bit confusing:  a horse is NOT considered "inbred" unless the duplicated ancestor appears in both its sire's and dam's pedigrees. For example, A.P. Indy is inbred to Bold Ruler. A daughter of A.P. Indy whose dam doesn't have Bold Ruler in her pedigree is not considered inbred to Bold Ruler.  Take a look at this example.

If A.P. Indy is crossed with a mare that does have Bold Ruler in her pedigree, we're back to calling the resulting foal "inbred to Bold Ruler."  Pulpit serves as a good example: he is inbred 5 x 4 x 5 to Bold Ruler. He's also inbred to Bold Ruler's sire, Nasrullah -- in this case, 6S x 6s x 5S x 5D x 6D. Note that Pulpit is considered inbred to Nasrullah because Nasrullah appears in the first five generations of both his sire's and dam's pedigrees and through different progeny.  If Nasrullah appeared only once within five generations but was duplicated further back in the pedigree he would be considered a line-bred influence; if he appeared only as the sire of Bold Ruler, he wouldn't be considered either an inbred or line-bred influence, no matter how many times or in what location Bold Ruler appears.

There are some inbreeding theories that are worth looking into. The duplicated sire line (where inbreeding occurs specifically in the direct sire line and the direct damsire line) is one we'll take a look at in an upcoming study. The Rasmussen Factor is a specific pattern of inbreeding to mares, and is one I'll discuss in detail in a future post.

So, if you've followed all that, here are a couple of interesting pedigrees to look at:

  1. Roberto (pedigree).  This is one of the most inbred pedigrees you're likely to find amongst modern sires. You'll see inbreeding to Nearco and Pharos and Mumtaz Begum and Plucky Liege and Blue Larkspur. There's additional line-breeding to Sir Gallahad (4s x 6D).  There's a duplication of Sardanapale -- but while Bramalea is considered inbred to him, note that Roberto is not!  Similarly, Hail to Reason's dam is inbred 3 x 5 to Man o'War, but Hail to Reason is not considered inbred to him, and Roberto (who has Man o'War 5s x 6s) is not considered line-bred to the great Big Red.
  2. Salambria (pedigree). This is one of my mares, and you'll notice she's inbred 4S x 4d to Intentionally and 5S x 4s x 4d to Aspidistra.  I have Salambria booked to Request for Parole for an April cover -- here is the hypo-mating pedigree.  This mating will create additional inbreeding to Secretariat (4s x 4d) and In Reality (4S x 4d), while expanding the inbreeding to Aspidistra (now 5s x 6D x 5d x 5d). 

 Here's your challenge, should you choose to accept:  based on standard notation, try to spot what's wrong with the hypothetical mating report for my 2010 foal out of Salambria.  Answers in the comments section, please. 

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