DETERMINING THE LINKAGE OF DISEASE-RESISTANCE GENES TO MOLECULAR MARKERS - THE LOD-SCORE METHOD REVISITED WITH REGARD TO NECESSARY SAMPLE SIZES

Some approaches to molecular marker-assisted linkage detection for a d ominant disease-resistance trait based on a segregating F-2 population are discussed. Analysis of two-point linkage is carried out by the tr aditional measure of maximum led score. It depends on (1) the maximum- likelihood estimate of the recombination fraction between the marker a nd the disease-resistance gene locus, (2) the observed absolute freque ncies, and (3) the unknown number of tested individuals. If one replac es the absolute frequencies by expressions depending on the unknown sa mple size and the maximum-likelihood estimate of recombination value, the conventional rule for significant linkage (maximum led score excee ds a given linkage threshold) can be resolved for the sample size. For each sub-population used for linkage analysis [susceptible (= recessi ve) individuals, resistant (= dominant) individuals, complete F-2] thi s approach gives a lower bound for the necessary number of individuals required for the detection of significant two-point linkage by the le d-score method.