A search for quantitative trait loci for milk production traits on chromosome 6 in Finnish Ayrshire cattle

Cattle chromosome 6 was scanned with 11 markers, ten microsatellites and th e casein haplotype, to identify quantitative trait loci (QTLs) affecting th e following milk production traits: milk yield, fat percentage, fat yield, protein percentage and protein yield. Twelve Finnish Ayrshire half-sib fami lies with a total of 480 sons were genotyped and used in a granddaughter de sign. Interval mapping was performed with a multiple-marker regression appr oach with a one-QTL and a two-QTL model, and the significance threshold val ues were determined empirically using a permutation test. Across-family ana lysis with the one-QTL model revealed an effect on protein percentage (P < 0.05) and on milk yield (P < 0.05). The analysis with the two-QTL model ide ntified significant effects (P < 0.05) on protein percentage, milk yield, a nd fat yield. Comparing these two cases, the results suggest the existence of two QTLs on chromosome 6 with an effect on milk production traits. One o f the QTLs was:located around the casein genes. As the other QTL was simila r in location and effect to a QTL found previously in Holstein-Friesians, a n identity-by-descent approach could be applied to fine map this region.