作者单位:NYU, Sch Med, Div Biostat, Dept Populat Hlth, New York, NY 10016 USA[1]Univ N Carolina, Dept Genet, Chapel Hill, NC 27599 USA[2]Univ N Carolina, Dept Biostat, Chapel Hill, NC 27599 USA[3]Nanjing Agr Univ, Coll Vet Med, Nanjing 210095, Jiangsu, Peoples R China[4]