A genetic study of twinning rate in Norwegian cattle / En genetisk studie av tvillingfødsler hos NRF
The main aim of this thesis was to increase the knowledge on twinning rate in Norwegian Cattle. The work has been based on recordings in the Norwegian Dairy Cattle Recording Service, where information on twin births has been recorded on a countrywide basis since 1978.
The thesis consists of a total of 5 papers. Paper I presents a review of twinning in cattle, and it is concluded that it is possible to select for increased twinning rate in cattle, and that this strategy could be of interest in an intensive-management system.
In Paper II variance components and breeding values for twinning rate were estimated. Records from approximately 1.4 million first parity and 0.9 million second parity cows and 2043 sires were analyzed in a linear sire model. The heritability of twinning rate in first and second parity was estimated to approximately 1 and 3% respectively. Both phenotypic and genetic trend showed a slight increase in the frequency of twin births from 1978 to 1995.
Paper III describes a quantitative trait loci (QTL) analysis for twinning rate in Norwegian Cattle. A total of 6 bull-sires and 285 sons with genotype information, and approximately 250.000 granddaughters with information on twinning rate were used in the analysis. A genome scan using a granddaughter design indicated putative QTL on chromosome 5, 7, 12, and 23.
Paper IV and V show fine mapping results for the QTL on chromosome 5 using linkage disequilibirum mapping (Paper IV) and combined linkage and linkage disequilibrium mapping (Paper V), respectively. The final results from Paper IV and Paper V showed that the QTL on chromosome 5 was positioned to an interval of approximately 3.5 and 1.3 Centimorgan, respectively.
