Rachel Alexandra and the Rasmussen Factor

A reader's comment to a post last week (Thoroughbred Pedigree Review of KEESEP Hip #4233 -- A Rasmussen Factor Case Study) pointed out the significant mare inbreeding in a 1952 mare named Ellen's Best (pedigree). Ellen's Best's fourth dam was Friar's Carse, who was also the dam of her sire War Relic. In turn, War Relic was inbred to Fairy Gold, the dam of both his grandsire and his damsire. (It makes more sense when you're looking at the pedigree!) 

Ellen's Best was herself an undistinguished runner -- she managed one show effort from 14 starts for $250 lifetime earnings -- but the family 1-o mare had a strong pedigree and is well-represented 50-odd years later. Not the least of her tail-female descent is Rachel Alexandra.

I responded to Archer's comment quickly by tracing Ellen's Best's line down to Rachel Alexandra and noting a handful of additional Rasmussen Factors (RFs) throughout Rachel's female lineage.  More, it would seem, than most female lines would boast.

Well, I decided to look back in Rachel Alexandra's full pedigree to see what other Rasmussen influences there would be.  Turns out, a lot.

Her dam, granddam, sixth dam, seventh dam, 10th dam, 14th dam, 17th dam, 21st dam, 22nd dam, and 23rd dam all had at least one RF -- that's 10 mares out of the first 23 in tail-female descent, or 43.5%.

I looked at the first six generations of Rachel Alexandra's pedigree and found that of 126 ancestors, 20 (15.9%) had an RF, with a higher percentage in closer generations. Here's a chart that shows the percentage of ancestors with an RF has mostly increased in later generations:

Gen. No. Ancestors
This Gen. / Cum.
Ancestors w/RF (%)
-- This Generation --
Ancestors w/RF (%)
-- Cumulative --
1 2 / 2 1 (50.0%) 1 (50.0%)
2 4 / 6 1 (25.0%) 2 (33.3%)
3 8 / 14 2 (25.0%) 4 (28.6%)
4 16 / 30 5 (31.3%) 9 (30.0%)
5 32 / 62 3 (9.4%) 12 (19.4%)
6 64 (126) 8 (12.5%) 20 (15.9%)

Four of those Rasmussen influences are to Canterbury Pilgrim (two through Nearctic and two through Lady Angela) and six are to Plucky Liege (through Sir Ivor, Stop the Music, Lord Gaylord, and three times through Hail to Reason).  Other RFs are to Selene (twice), Alcibiades, Black Duchess, Briar-Root, Dubbed In, Herodias, Imperatrice, Lavendula, Pocohontas, and Sanda.  Only Lord Gaylord (Rachel's third damsire) had two RFs.  (But note that Rachel's 23rd dam, Promise (pedigree), had three Rasmussen Factor inbreedings plus a fourth mare doubled through her dam!)

I'm not aware of any valid studies that identify the percentage of the Thoroughbred population -- current or historic -- that have Rasmussen inbreeding. But it's probably around 2 to 3% for all Thoroughbreds and maybe 3 to 6% for what most people would consider "well-bred" horses. Rachel's six-generation cumulative total of 16% ancestors with RF is likely several times what would be seen in the average filly born in 2006. And certainly 30% RF ancestors through four generations has got to be anomalous.

Now, a single case doesn't make a good argument and there's a whole lot more going on in Rachel's pedigree than female family inbreeding, but it is fascinating to consider how much of an influence mare duplications have played in the development of today's hottest filly.

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