Evaluation of sire predicted transmitting abilities for evidence of X-chromosomal inheritance in North American sire families

This study tested for differences between genetic merits of sons and daught ers of sires and for evidence of segregating quantitative trait loci on the X chromosomes of North American Holsteins. Son PTA adjusted for sire PTA w as used as the dependent variable to test for biases and for genes that wer e passed from sire to daughter but not to son. The test of variability acro ss sires of sons merely indicated an unaccounted source of variation, for w hich genes on X chromosomes might be responsible. Critical values for this test and power were determined by simulation for a variety of populations a nd traits differing in heritability, size of the X chromosome effect, and a llelic frequency. Simulated genes on the X chromosome were detected with hi gh power at intermediate frequencies of the favorable allele. The power of the test increased as the size of the effect increased and as genetic varia nce attributed to autosomes decreased. The test was then applied to recentl y evaluated data from US and Canadian Holstein populations. Genetic evaluat ions for >17,000 bulls from the US and >9000 from Canada were included. Res ults suggested that little extra variation was present for some traits form ally evaluated in North America, but that genes on the X chromosome were un likely to be the cause.