Northern Dancer (May 27, 1961 - November 16, 1990) was a Canadian-bred Thoroughbred racehorse that won the 1964 Kentucky Derby and Preakness Stakes and then became one of the most successful sires of the 20th century. He is considered a Canadian icon, and was inducted into the Canadian Sports Hall of Fame in 1965. Induction into the Racing Hall of Fame in both Canada and the United States followed in 1976.
Because of the prevalence of Northern Dancer's bloodline, a certain degree of inbreeding to him is becoming common, especially in Europe and Australia. 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. Statistical studies have shown that inbreeding has increased by a small but significant amount in the last 50 years, partly because of the larger number of foals that a successful stallion will now sire each year compared to in the past. To illustrate this last point, Northern Dancer produced 645 foals in 20 years at stud, whereas his grandson Danehill sired 2,499 foals in 14 years at stud. Statistical analysis has shown that inbreeding to Northern Dancer is on average slightly less effective than when stallions of his line are bred to mares who do not have Northern Dancer in their pedigree (referred to as an outcross). However, many successful stallions are inbred, in part because this can make it easier to pass on dominant characteristics. Notable sires that are inbred to Northern Dancer (within four generations) include Oasis Dream, Rock of Gibraltar, Hernando, Spinning World, and Redoute's Choice. The undefeated Frankel is also inbred to Northern Dancer, but it is too early to evaluate his success as a stallion as his first crop just reached racing age in 2016. Analysis of inbreeding to Northern Dancer has shown that best practice is inbreeding "through" the best, which means to use horses that have demonstrated their fitness on the racecourse. In contrast, inbreeding to a descendant such as Danzig is viewed as potentially problematic because he was retired from racing due to unsoundness after only three starts. However, Danzig line horses have been crossed successfully with other Northern Dancer line horses, especially Sadlers' Wells. Outcross bloodlines can still be found, most notably via the German-bred stallion Monsun. Northern Dancer lines were originally outcrossed on descendants of Mr. Prospector or Nasrullah, but this had been done to such as extent that it is increasingly difficult to find horses from these lines without some trace of Northern Dancer breeding. In North America, Northern Dancer often appears further back in the pedigree of major stallions, which reduces the risks associated with inbreeding. For example, Tapit, the leading sire in North America for 2014-2015, is inbred to Nijinsky 3 x 5 (thus only 4 x 6 to Northern Dancer) and has crossed well with mares from other Northern Dancer lines.

Q: Are there any other interesting aspects about this article?
A: Because of the prevalence of Northern Dancer's bloodline, a certain degree of inbreeding to him is becoming common,

Q: How did imbreedng hurt the horses?
A: there is a concern that the inbreeding will weaken the breed, making horses more prone to injury and potentially leading to inbreeding depression.
were they breeding him with his chrildren?
slightly less effective than when stallions of his line are bred to mares who do not have Northern Dancer in their pedigree many successful stallions are inbred, Northern Dancer produced 645 foals in 20 years at stud, whereas his grandson Danehill sired 2,499 foals in 14 years at stud. his grandson Danehill sired 2,499 foals in 14 years at stud.
his grandson Danehill sired 2,499 foals in 14 years at stud.