GENETIC-MAPPING OF QUANTITATIVE TRAIT LOCI FOR TRAITS WITH ORDINAL DISTRIBUTIONS

Statistical methods for mapping quantitative trait loci relative to ge netic markers are now well established for continuous traits with norm al distributions. However, many traits of economic importance are reco rded on a discrete, ordinal scale. Here we describe a model developed for the analysis of ordinal traits, such as degree of difficulty in ca lving or categories of plant disease resistance. The model estimates t he distance from the quantitative trait locus to neighbouring genetic markers, and also genetic parameters, either as gene effects on an und erlying continuous scale or as probabilities of the observed categorie s. The model is tested on simulated data and is compared with an analy sis based on mixtures of normal distributions. The ordinal model is fo und to estimate the parameters more accurately, especially when the nu mber of categories is small or when only one linked marker is availabl e.