G C A T T A C G G C A T genes Article A Large-Scale Genome-Wide Association Study of Epistasis Effects of Production Traits and Daughter Pregnancy Rate in U.S. Holstein Cattle Dzianis Prakapenka 1, Zuoxiang Liang 1, Jicai Jiang 2, Li Ma 3 and Yang Da 1,* 1 Department of Animal Science, University of Minnesota, Saint Paul, MN 55108, USA;
[email protected] (D.P.);
[email protected] (Z.L.) 2 Department of Animal Science, North Carolina State University, Raleigh, NC 27695, USA;
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[email protected] * Correspondence:
[email protected] Abstract: Epistasis is widely considered important, but epistasis studies lag those of SNP effects. Genome-wide association study (GWAS) using 76,109 SNPs and 294,079 first-lactation Holstein cows was conducted for testing pairwise epistasis effects of five production traits and three fertility traits: milk yield (MY), fat yield (FY), protein yield (PY), fat percentage (FPC), protein percentage (PPC), and daughter pregnancy rate (DPR). Among the top 50,000 pairwise epistasis effects of each trait, the five production traits had large chromosome regions with intra-chromosome epistasis. The percentage of inter-chromosome epistasis effects was 1.9% for FPC, 1.6% for PPC, 10.6% for MY, 29.9% for FY, 39.3% for PY, and 84.2% for DPR. Of the 50,000 epistasis effects, the number of significant effects with log10(1/p) ≥ 12 was 50,000 for FPC and PPC, and 10,508, 4763, 4637 and 1 for MY, FY, PY and DPR, Citation: Prakapenka, D.; Liang, Z.; respectively, and A × A effects were the most frequent epistasis effects for all traits.