Number of ancestors explaining 50, 75, and 99% of the gene pool seen in 26 cohorts of control (C) or affected (A) racehorses.

<p>Number of ancestors explaining 50, 75, and 99% of the gene pool seen in 26 cohorts of control (C) or affected (A) racehorses.</p

Percentage of affected horses which have each of 11 significant ancestors present at least once in their five-generation pedigree.

<p>Percentage of affected horses which have each of 11 significant ancestors present at least once in their five-generation pedigree.</p

Marginal ancestral contributions to atrial fibrillation in the Standardbred racehorse: Comparison of cases and controls

<div><p>Admissions of Standardbred racehorses (<b>Std</b>) to the Ontario Veterinary College Teaching Hospital (<b>OVCTH</b>) for treatment of atrial fibrillation (<b>AF</b>) began to increase in the early 1990s. The arrhythmia has been shown to have a modest heritability (h² ≃ 0.15), with some stallions appearing as sires or sires of mares used in breeding (broodmares) of affected horses more frequently than others. The objective of this study was to determine the marginal genetic contributions of ancestors to cohorts of Std affected with AF and their contemporary control groups, and whether these ancestors contribute significantly more to the affected cohorts than to controls. All Std admitted to OVCTH for treatment of AF that were born between 1993 and 2007 comprised the affected case group (n = 168). Five randomly selected racing contemporaries for each Std admitted, assumed to not suffer from the arrhythmia, comprised the control group (n = 840). Three-year overlapping cohorts were created for case and control horses, determined according to year of birth, for a total of 26 cohorts. Marginal genetic contributions of ancestors to each cohort were determined and differences analyzed for statistical significance using a two-tailed paired t-test, with P ≤ 0.05 considered significant. The marginal contributions of 26 ancestors were significant, with 11 contributing significantly more to affected cohorts than the corresponding controls, and 15 contributing significantly more to controls than the corresponding affected cohorts. One stallion and one broodmare were very highly significant to affected cohorts at P ≤ 0.001, and nine stallions and three broodmares were very highly significant to control cohorts at P ≤ 0.001. Therefore, a number of stallions have statistically significant contributions to the genetics of Std affected with AF, while many others have statistically significant contributions to healthy Std. The arrhythmia appears to be particularly prevalent in the descendants of one sire family.</p></div

Marginal genetic contributions of the five highest-contributing ancestors significant to control (Fig 1A) and affected (Fig 1B) cohorts, selected according to largest average non-zero contribution.

<p>Marginal genetic contributions of the five highest-contributing ancestors significant to control (Fig 1A) and affected (Fig 1B) cohorts, selected according to largest average non-zero contribution.</p

Diagram depicting the inter-relationships between eight of 11 ancestors significant to affected cohorts (outlined in purple) and one ancestor with affected offspring that is also a sire and broodmare sire of additional affected offspring (outlined in black), with ancestors in the pathways indicated by P. Sire lines are presented in pink, with dam lines in blue.

<p>Diagram depicting the inter-relationships between eight of 11 ancestors significant to affected cohorts (outlined in purple) and one ancestor with affected offspring that is also a sire and broodmare sire of additional affected offspring (outlined in black), with ancestors in the pathways indicated by P. Sire lines are presented in pink, with dam lines in blue.</p

Pairwise two-tailed t-tests for the 41 ancestors with the highest average marginal contributions to two or more non-overlapping cohorts.

<p>Pairwise two-tailed t-tests for the 41 ancestors with the highest average marginal contributions to two or more non-overlapping cohorts.</p

Marginal genetic contributions to three-year cohorts of control horses from 41 highest-contributing ancestors (Anc.) and overall average contribution (Ave.).

<p>Marginal genetic contributions to three-year cohorts of control horses from 41 highest-contributing ancestors (Anc.) and overall average contribution (Ave.).</p

Descriptive pedigree statistics for each affected and control cohort showing average, maximum, and minimum inbreeding, number of sires, number of dams, average genetic relationship, and average 5-generation pedigree completeness index (PCI).

<p>Descriptive pedigree statistics for each affected and control cohort showing average, maximum, and minimum inbreeding, number of sires, number of dams, average genetic relationship, and average 5-generation pedigree completeness index (PCI).</p

Marginal genetic contributions to three-year cohorts of affected horses from 41 highest-contributing ancestors and overall average contribution.

<p>Marginal genetic contributions to three-year cohorts of affected horses from 41 highest-contributing ancestors and overall average contribution.</p

Number of five-generation pedigrees of affected Standardbred racehorses in which three significant ancestors appear once or multiple times and through the sire or dam of the horse, or through both.

<p>Number of five-generation pedigrees of affected Standardbred racehorses in which three significant ancestors appear once or multiple times and through the sire or dam of the horse, or through both.</p