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 daughters of sires and for evidence of segregating quantitative trait loci on the X chromosomes of North American Holsteins. Son PTA adjusted for sire PTA was used as the dependent variable to test for biases and for genes that were passed from sire to daughter but not to son. The test of variability across sires of sons merely indicated an unaccounted source of variation, for which genes on X chromosomes might be responsible. Critical values for this test and power were determined by simulation for a variety of populations and traits differing in heritability, size of the X chromosome effect, and allelic frequency. Simulated genes on the X chromosome were detected with high power at intermediate frequencies of the favorable allele. The power of the test increased as the size of the effect increased and as genetic variance attributed to autosomes decreased. The test was then applied to recently evaluated data from US and Canadian Holstein populations. Genetic evaluations for >17,000 bulls from the US and >9000 from Canada were included. Results suggested that little extra variation was present for some traits formally evaluated in North America, but that genes on the X chromosome were unlikely to be the cause.