The American Quarter Horse: Population Structure and Relationship to the Thoroughbred

A breed known for its versatility, the American Quarter Horse (QH), is increasingly bred for performance in specific disciplines. The impact of selective breeding on the diversity and structure of the QH breed was evaluated using pedigree analysis and genome-wide SNP data from horses representing 6 performance groups (halter, western pleasure, reining, working cow, cutting, and racing). Genotype data (36 037 single nucleotide polymorphisms [SNPs]) from 36 Thoroughbreds were also evaluated with those from the 132 performing QHs to evaluate the Thoroughbreds influence on QH diversity. Results showed significant population structure among all QH performance groups excepting the comparison between the cutting and working cow horses; divergence was greatest between the cutting and racing QHs, the latter of which had a large contribution of Thoroughbred ancestry. Significant coancestry and the potential for inbreeding exist within performance groups, especially when considering the elite performers. Relatedness within performance groups is increasing with popular sires contributing disproportionate levels of variation to each discipline. Expected heterozygosity, inbreeding, F-ST, cluster, and haplotype analyses suggest these QHs can be broadly classified into 3 categories: stock, racing, and pleasure/halter. Although the QH breed as a whole contains substantial genetic diversity, current breeding practices have resulted in this variation being sequestered into subpopulations.