The great-grand-daughter design: a simple strategy to increase the power of a grand-daughter design for QTL mapping

In dairy cattle, quantitative trait loci (QTL) are usually mapped using the grand-daughter design (GDD), i.e. sets of progeny-tested paternal half-bro thers. Linkage information is typically extracted from the segregation of t he sire chromosomes amongst their sons. We herein propose to increase the p ower of a GDD by exploiting the frequently occurring relationship between s ires and grandsons which has so far been ignored in most methods of analysi s. The proposed approach is a multipoint interval mapping method based on t he Wilcoxon sum-of-rank test. Three alternative approaches to combine infor mation from sons and grandsons are evaluated by simulation. In these either (i) sons and grandsons are ranked separately, (ii) sons and grandsons are ranked separately but the sign of the QTL effect is constrained to be the s ame in both generations, or (iii) sons and grandsons are ranked jointly. Th e proposed methods have been applied on a real data-set in which a GDD incl uding 907 sons is analysed with a marker map comprising nine microsatellite s spanning 46 cM on bovine chromosome 6.