Horse running through field

Inbreeding to a Sire in the Female Line

by Roger Lyons

Sid Fernando recently posted on Eskendreya at his Sid Fernando + Observations blog, and he included a link to something he posted on Eskendereya at WTC’s Who’s Hot blog just after the Fountain of Youth, and I went back and read it again. In that earlier post Sid draws a parallel between the pedigrees of Eskendereya and Real Quiet. What do they have in common? Inbreeding to a sire in the female line.

That way of putting it is easier than saying “inbreeding to the sire of a mare in the female line,” which is more accurate. I recall that many years ago David Dink, in his broad study of inbreeding for the Thoroughbred Times, chose the simpler expression, so I’ll use it, too. David did a study so broad that it was almost guaranteed not to find any effects of inbreeding, but, as I recall, he did a special installment on inbreeding to a sire in the female line because it was the only pattern of inbreeding that actually did get results that exceeded opportunity.

Just before re-reading Sid’s post from late February, I had posted some comments on Eskendereya (my last post), among which was the observation that Giant’s Causeway really didn’t have a very good strike rate with Northern Dancer, that he liked Raise a Native a lot more, and that’s true. However, after re-reading Sid’s post, which was about inbreeding to Northern Dancer in the female line of Eskendereya, I checked the numbers for occurrences of Northern Dancer as a sire in the female line of mares that have produced foals by Giant’s Causeway. Sid is going to like what I found.

Giant’s Causeway had 12 mares whose dams were by Northern Dancer–inbred 4×3 to Northern Dancer. The only mare to produce a superior runner was the dam of Aragorn, a dual-G1 winner. The story is high quality, low frequency, but that’s not the end of it.

He sired foals out of 16 mares whose third dams were by Norther Dancer–inbred 4×4 to Northern Dancer. Two of those mares produced superior runners, including G1 winner Frost Giant and Model, a listed stakes winner. It seems fair to assess that provisionally as high quality, average frequency, especially since it’s Eskendereya’s pattern. He won’t be included in my tallies until after the classics.

Giant’s Causeway sired foals out of six mares whose fourth dams were by Northern Dancer–inbred 4×5 to Northern Dancer. Two of those mares produced G1 winners, including Red Giant and Internallyflawless. Unequivocally, that generational distance yielded high quality, high frequency.

Clearly, as far as Giant’s Causeway is concerned, there is something special about mares with Northern Dancer as a sire in the female line, and I wouldn’t be surprised to find that this approach does have broad effects for certain sires, as David Dink’s study found and as is suggested by Sid’s comparison. After all, Raise a Native is the key sire in Real Quiet’s female line. It’s not just a Northern Dancer thing, but generational distance could be a factor, especially in regard to frequency.

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