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OK so the sire isn't properly line-bred because the maternal and paternal sire are the same. I don't get why they aren't the best line-bred?


That particular pattern of "linebreeding" has proven not to be the best. In the colt and filly factors post, it explains the sibling relationship crosses that have been proven to be most successful. Yes, that is an opposite sex sibling cross in the pedigree, but it's all on one side. If there were two more crosses to that horse on the bottom in that subject horse's pedigree (through different descendants), then that ancestor might be more influential.