Editing Northern Dancer (section)

===Inbreeding===
Because of the prevalence of Northern Dancer's bloodline, a certain degree of [[inbreeding]] to him is becoming common.<ref name=DoublingDown /> 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.<ref>{{cite web|last1=Walker|first1=Matt|title=Wonder Monkey: Are racehorses being bred to destruction?|url=https://www.bbc.co.uk/blogs/wondermonkey/2011/11/bred-to-destruction.shtml|website=BBC|access-date=9 July 2016|date=November 18, 2011}}</ref> For example, Northern Dancer sired 645 foals<ref name="ACP" /> in 20 years at stud, whereas his grandson Danehill sired 2,499 foals in 14 years at stud.<ref name="Danehill">{{cite web|last1=Sparkman|first1=John P.|title=Pedigree Analysis: American Roots|url=http://www.bloodhorse.com/horse-racing/articles/121903/pedigree-analysis-american-roots|website=BloodHorse|access-date=9 July 2016|date=April 9, 2013}}</ref>

[[File:Frankel (horse).jpg|thumb|The undefeated Frankel is inbred to Northern Dancer.]]
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]]).<ref name="DoublingDown">{{cite web|title=Pedigree Analysis: Doubling Down On Danzig|url=http://www.bloodhorse.com/horse-racing/articles/106095/pedigree-analysis-doubling-down-on-danzig|website=BloodHorse|access-date=9 July 2016|date=August 17, 2015}}</ref> However, many successful stallions are inbred, in part because this can make it easier to pass on [[dominant gene|dominant]] characteristics. Notable sires that are inbred to Northern Dancer (within four generations) include [[Oasis Dream]], [[Rock of Gibraltar (horse)|Rock of Gibraltar]], [[Hernando (horse)|Hernando]], [[Spinning World]], [[Redoute's Choice]], and [[Frankel (horse)|Frankel]].<ref name="LineBreeding">{{cite web|last1=Porter|first1=Alan|title=Line-breeding to Northern Dancer and Nasrullah in Thoroughbred Stallion Pedigrees|url=http://cs.bloodhorse.com/blogs/truenicks/archive/2008/11/30/line-breeding-to-northern-dancer-and-nasrullah-in-thoroughbred-stallion-pedigrees.aspx|website=TrueNicks|access-date=9 July 2016|date=November 30, 2008}}</ref><ref>{{cite web |last1=Berry |first1=John |title=Frankel A Rising Star At Stud |url=https://www.thoroughbreddailynews.com/frankel-a-rising-star-at-stud/ |website=Thoroughbred Daily News {{!}} Horse Racing News, Results and Video {{!}} Thoroughbred Breeding and Auctions |access-date=4 July 2021 |language=en |date=28 November 2018}}</ref>

Northern Dancer lines were originally outcrossed on descendants of [[Mr. Prospector]] or [[Nasrullah (horse)|Nasrullah]], but this became so common that it is increasingly difficult to find horses from these lines who do not also carry Northern Dancer breeding.<ref>{{cite web|title=Is Sire-Line Outcrossing Possible Anymore?|url=http://cs.bloodhorse.com/blogs/scot/archive/2008/06/22/is-sire-line-outcrossing-possible-anymore.aspx|website=cs.bloodhorse.com|access-date=9 July 2016|date=June 22, 2008}}</ref> Outcross bloodlines can still be found, most notably through [[Sunday Silence]]<ref>{{cite web |last1=MacDonald |first1=Michele |title=These new elite stallions show how Northern Dancer and Mr Prospector still rule the world |url=https://www.thoroughbredracing.com/articles/these-new-elite-stallions-show-how-northern-dancer-and-mr-prospector-still-rule-world/ |website=Thoroughbred Racing Commentary |access-date=3 August 2019|date=February 16, 2018}}</ref> and the German-bred stallion [[Monsun]].<ref>{{cite web|last1=Porter|first1=Alan|title=Monsun Season Underway in Europe|url=http://cs.bloodhorse.com/blogs/truenicks/archive/2013/06/04/monsun-season-underway-in-europe.aspx|website=TrueNicks|access-date=9 July 2016|date=June 4, 2013}}</ref>

In North America, Northern Dancer is farther back in the pedigree of most major sires, decreasing the risks associated with inbreeding. For example, [[Tapit]], the leading sire in North America from 2014 to 2016, is inbred 3 × 5 to [[Nijinsky II]] (thus 4 × 6 to Northern Dancer). He has crossed well with mares from other Northern Dancer lines.<ref>{{cite web|last1=Peters|first1=Anne|title=The Secret to Tapit's Success|url=http://www.bloodhorse.com/horse-racing/articles/122501/the-secret-to-tapits-success|website=BloodHorse|access-date=9 July 2016|date=March 12, 2013}}</ref>