| Twinning in dairy cattle has been associated with many negative health and reproductive events that cause economic loss to the producer. Reports have suggested that twinning rates are increasing and that there may be a positive relationship between milk production and twinning frequency. Quantitative trait loci (QTL) for twinning rate on bovine chromosomes 5, 7, 19, and 23 have been previously identified in other populations. The objectives of this study were to detect and confirm the existence and effects of these QTL and to look for QTL for milk yield, fat and protein yield and percentage, somatic cell score, and productive life in those same chromosomal regions. Half-sib families of 20 North American Holstein sires, patriarchs, with high twinning rate PTA comprised the population under investigation. This project utilized sire predicted transmitting ability (PTA) values for the production traits from US Department of Agriculture. Twinning rate PTA values had been estimated from calving data. DNA extracted from semen samples was analyzed using 45 to 61 microsatellite markers across the four chromosomes. Marker heterozygosity of the patriarchs averaged 62%. Evidence of twinning QTL was found in multiple families on chromosomes 5, 7, and 23 and in one family on chromosome 19. Four of the patriarchs formed one three-generation family: one sire and three half-sib sons. This extended family was analyzed with additional markers confirming a twinning QTL on chromosome 5; the magnitude of the effect of the QTL was a 0.7% change in twinning rate. Production QTL were found on chromosomes 5 and 7 for milk; chromosomes 5 and 7 for fat yield; chromosomes 5, 7, 19, and 23 for fat percentage; chromosome 7 for protein yield; chromosome 5 for protein percentage; chromosomes 19 and 23 for productive life, and chromosomes 5, 19, and 23 for somatic cell score. |
