Power for mapping quantitative trait loci in crosses between outbred lines in pigs

Power for mapping quantitative trait loci using crosses between segregat- ing populations was studied in pigs. Crossing generates gametic disequilibrium and increases heterozygosity. The condition for a heterozygosity among F individuals to be greater than in either line at crossing is that allele frequency should be lower than 1/2 in one line and higher than 1/2 in the other line. Maximum expected power and expected risk were used to compare hierarchical backcross (each boar mated to several sows; contrast within boars), hierarchical intercross (each boar mated to several sows; contrast within both boars and sows) and traditional intercross (each boar mated to one sow; contrast within both boars and sows). The use of hierarchical designs (back- cross and intercross) increased power for traits with low or intermediate heritabilities. For small QTL effects and low heritabilities the hierarchical backcross design gave the highest expected power but also the highest risk. There is not a general design which allocates resources in an optimum fashion across situations (heritabilities, QTL ef- fect, heterozygosity). A compromise between designs with high power and low risk is suggested. A hierarchical backcross design of 400 piglets and heterozygosity 0.68 requires between four (maximum expected power) and eight boars (minimum risk) to detect a QTL of 0.5 phenotypic standard deviations. Selection of extreme individuals in parental lines increased power up to 21 %. Commercial crosses are proposed as an alternative to experiments for QTL mapping. @ Inra/Elsevier, Paris gene mapping / quantitative trait locus / statistical power / hierarchical design / pig