Conversation so far:
Are there any other interesting aspects about the Northern Dancer article?
Because of the prevalence of Northern Dancer's bloodline, a certain degree of inbreeding to him is becoming common.
How does inbreedng hurt race horses?
Whenever this happens, there is a concern that the inbreeding will weaken the breed, making horses more prone to injury and potentially leading to inbreeding depression.

Question: were they breeding him with his chrildren?
Self-contained rewrite: Were breeders breeding Northern Dancer with his chrildren?