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Question from Shimatoree:  Would you please comment on the influence of inbreeding ( or line-breeding) to Northern Dancer and Nasrullah in stallion pedigrees.

Alan's reply:  That is a difficult question to reply to in general terms. Historically, what I have seen happen is that inbreeding to a famous horse is an emotive issue when it first appears, but after a while begins to be taken for granted. To give an example, in a book written around 1930 -- The Breed of The Racehorse by Becker -- fear is expressed that inbreeding to St. Simon (the greatest stallion of the late 19th century) is saturating the breed. Incidentally, Nearco, the sire of Nasrullah and grandsire of Northern Dancer, has four crosses of St. Simon in the first four generations of his pedigree (5 x 4 x 4 x 5) and a sister to St. Simon in the seventh.

Studies have tended to show inbreeding to any specific degree tends to be similar across the population of stakes winners and the population as a whole (the top commercial population tends to have a higher degree of inbreeding and line-breeding, as they are drawn from a smaller gene pool). What tends to have a positive influence is inbreeding in certain positions, and inbreeding through related strains -- brothers, sisters, half-siblings from the same sire line, and other crosses that are similar but less easily defined in terms of human relationships.
 
In most modern stallions, line-breeding to Nasrullah is probably too far back to have an obvious influence (which is not to say that accumulations of Nasrullah, his sisters Rivaz and Malindi; the similarly-bred Royal Charger; and relatives such as Mahmoud, Badruddin (broodmare sire of My Babu)  might not have an effect). Stallions with a Northern Dancer strain through horses that have relatives, such as Nijinsky II, his genetic relative The Minstrel (himself a relative to Vice Regent and Viceregal) and Far North, the similarly-bred Storm Bird, have a great opportunity to combine in interesting matings.

Successful sires that are inbred to Northern Dancer (within four generations) include Hernando, Oasis Dream, Rock of Gibraltar, Flying Spur, Spinning World, Touch Gold, and Redoute's Choice. Rock of Gibraltar, Flying Spur, and Redoute's Choice are all by Danehill, who brings in a half-sister to Northern Dancer.
 
I closing, I'll mention that I did see an article by Jay Leimbach that presented an interesting argument suggesting that leading sires are often inbred to a male line ancestor - for example, Seattle Slew, who carried on the Bold Ruler line, is inbred to Bold Ruler's sire, Nasrullah.