Abstract

Most phenotypes in human and animals have a multifactorial background, e.g. they depend on many different genes and environmental factors may play a prominent role. Geneticists have long been concerned with identifying key genes responsible for variation in multifactorial traits, such as obesity and diabetes in humans and production traits in animals. In this thesis I have used two different intercrosses to map Quantitative Trait Loci (QTL) for growth and production traits in chicken. The first part of the thesis is based on an intercross between White Leghorn and the ancestor of the modern domesticated chicken, the red junglefowl. A total of 13 QTLs reached genome-wide significance and the four major QTLs explain around 80% of the phenotypic variance between the parental populations in males, indicating that a few QTLs have had a large influence on the enhanced growth rates in domesticated chickens. The second intercross is between two chicken lines divergently selected for body weight at 56 days of age. The selection response has been remarkable and after 42 generations of selection the lines differ almost nine-fold in weight. QTL studies revealed 13 QTLs for growth and each QTL explains a small part of the phenotypic variance within the F2 generation. Altogether, the 13 QTLs explain a smaller part of the population variance compared to the red junglefowl x White Leghorn intercross. For each QTL, the allele from the high line was associated with enhanced growth. Finally, the gene coding for melanocortin receptor 3 (MC3R) was evaluated as a positional candidate gene for an early growth QTL on chromosome 20 in chicken. The analysis showed that the high and low lines are fixed for different MC3R alleles, an observation that strengthens MC3R as a positional candidate gene. Expression analysis revealed a significant differential expression with higher expression in the low line at hatch. Further analyses indicated that this differential expression was primarily due to trans-acting factor(s). The two large QTL studies presented in the thesis has potential to result in identification of causative trait nucleotides for production traits in chickens as well as interesting candidate target genes for human metabolic disorders.