Quantitative genetic analysis of ascites in broilers

Ascites, also called pulmonary hypertension syndrome, is a metabolic disorder in chickens. The tendency to develop ascites is heritable, and successful selection will benefit from understanding the genetic background of ascites. The aims of this thesis were to (i) estimate genetic parameters for ascites indicator traits using standard and novel statistical methods and (ii) to perform a genome wide association study in order to identify chromosomal regions associated with ascites. It was shown that blood gas parameters, which have been previously suggested as indicator traits for ascites susceptibility, are of limited value in the selection against ascites susceptibility. Further, it was shown that the incidence of ascites is higher in male than in female broilers and this difference affects the genetic parameters.A liability normal mixture (LNM) model can account for differences in incidence of ascites. Analyses using the LNM model showed that almost a quarter of the chickens in the analyzed population are affected by ascites. Further, this analysis showed that the ratio of right to total ventricular weight (RATIO) is genetically not the same trait in healthy and diseased chickens. Finally, the results of a genome wide association study (GWAS) showed a number of chromosomal regions associated with ascites. No regions were detected with major effects on ascites. Therefore it seems that ascites is affected by a number of genes, each with a relatively small effect