In vivo haploid induction (HI) triggered by pollination with special intraspecific genotypes, called inducers, is unique to Zea mays L. within the plant kingdom and has revolutionized maize breeding during the last decade. However, the molecular mechanisms underlying HI in maize are still unclear. To investigate the genetic basis of HI, we developed a new approach for genome-wide association studies (GWAS), termed conditional haplotype extension (CHE) test that allows detection of selective sweeps even under almost perfect confounding of population structure and trait expression. Here, we applied this test to identify genomic regions required for HI expression and dissected the combined support interval (50.34 Mb) of the QTL qhir1, detected in a previous study, into two closely linked genomic segments relevant for HI expression. The first, termed qhir11 (0.54 Mb), comprises an already fine-mapped region but was not diagnostic for differentiating inducers and noninducers. The second segment, termed qhir12 (3.97 Mb), had a haplotype allele common to all 53 inducer lines but not found in any of the 1482 noninducers. By comparing resequencing data of one inducer with 14 noninducers, we detected in the qhir12 region three candidate genes involved in DNA or amino acid binding, however, none for qhir11. We propose that the CHE test can be utilized in introgression breeding and different fields of genetics to detect selective sweeps in heterogeneous genetic backgrounds.KEYWORDS in vivo haploid induction; selective sweep; genome-wide association study; population structure; Zea mays L T HE double haploid (DH) technology based on in vivo haploid induction (HI) has become one of the most important tools in maize breeding during the past decade and is replacing the conventional method of line development by recurrent selfing . The success of this new technology became possible, because dozens of maize inducer lines have been developed worldwide (reviewed in Supplemental Material, File S1) which, when used as pollinators, trigger the production of seeds with haploid embryo at an acceptable rate, i.e., .2% . Double fertilization followed by elimination of the inducer chromosomes in the embryo at later developmental stages (Li et al. 2009;Xu et al. 2013) as well as parthenogenesis (Sarkar and Coe 1966;Beckert et al. 2008) HI in maize, but a proof of these hypotheses requires profound knowledge about the genetic and physiological factors underlying this phenomenon. All previous QTL mapping studies for unraveling the genetic architecture of HI detected a major QTL on chromosome 1 (Röber 1999;Beckert et al. 2008; Prigge et al. 2012). The most comprehensive study with four biparental populations (Prigge et al. 2012) mapped this QTL, termed qhir1, to bin 1.04 and hypothesized that it is required for HI, but QTL positions and 1-LOD support intervals differed substantially among populations. In another study with population 1680 3 UH400, Dong et al. (2013) fine mapped a 3.57-Mb region between markers umc1917 and bnlg1811, w...