Methods are presented for determining linkage between a marker locus and a nearby locus affecting a quantitative trait (quantitative trait locus=QTL), based on changes in the marker allele frequencies in selection lines derived from the F-2 of a cross between inbred lines, or in the "high" and "low" phenotypic classes of an F-2 or BC population. The power of such trait-based (TB) analyses was evaluated and compared with that of methods for determining linkage based on the mean quantitative trait value of marker genotypes in F-2 or BC populations [marker-based (MB) analyses]. TB analyses can be utilized for marker-QTL linkage determination in situations where the MB analysis is not applicable, including analysis of polygenic resistance traits where only a part of the population survives exposure to the Stressor and analysis of marker-allele frequency changes in selection lines. TB analyses may be a useful alternative to MB analyses when interest is centered on a single quantitative trait only and costs of scoring for markers are high compared with costs of raising and obtaining quantitative trait information on F-2 or BC individuals. In this case, a TB analysis will enable equivalent power to be obtained with fewer individuals scored for the marker, but more individuals scored for the quantitative trait. MB analyses remain the method of choice when more than one quantitative trait is to be analyzed in a given population.

Recently a new class of genetic polymorphism, restriction fragment length polymorphisms (RFLPs), has been uncovered by the use of restriction endonucleases which cleave DNA molecules at specific sites and cloned DNA probes which detect specific homologous DNA fragments. RFLPs promise to be exceedingly numerous and are expected to have genetic characteristics - lack of dominance, multiple allelic forms and absence of pleiotropic effects on economic traits - of particular usefulness in breeding programs. The nature of RFLPs and the methodologies involved in their detection are described and estimated costs per polymorphism determination are derived. The anticipated costs of applying RFLPs to genome mapping are considered in terms of the number of RFLPs required for a given degree of genome coverage, the number of probe × enzyme combinations tested per polymorphism uncovered, and the total number of individuals and polymorphisms scored for mapping purposes. The anticipated costs of applying RFLPs to genetic improvement are considered in terms of the number of individuals and the number of polymorphisms per individual that are scored for the various applications. Applications considered include: varietal identification, identification and mapping of quantitative trait loci, screening genetic resource strains for useful quantitative trait alleles and their marker-assisted introgression from resource strain to commercial variety, and marker-assited early selection of recombinant inbred lines in plant pedigree breeding programs and of young sires in dairy cattle improvement programs. In most cases anticipated costs appear to be commensurate with the scientific or economic value of the application.

Selective DNA pooling (SDP) is a cost-effective means for an initial scan for linkage between marker and quantitative trait loci (QTL) in suitable populations. The method is based on scoring marker allele frequencies in DNA pools from the tails of the population trait distribution. Various analytical approaches have been proposed for QTL detection using data on multiple families with SDP analysis. This article presents a new experimental procedure, fractioned-pool design (FPD), aimed to increase the reliability of SDP mapping results, by ''fractioning'' the tails of the population distribution into independent subpools. FPD is a conceptual and structural modification of SDP that allows for the first time the use of permutation tests for QTL detection rather than relying on presumed asymptotic distributions of the test statistics. For situations of family and cross mapping design we propose a spectrum of new tools for QTL mapping in FPD that were previously possible only with individual genotyping. These include: joint analysis of multiple families and multiple markers across a chromosome, even when the marker loci are only partly shared among families; detection of families segregating (heterozygous) for the QTL; estimation of confidence intervals for the QTL position; and analysis of multiple-linked QTL. These new advantages are of special importance for pooling analysis with SNP chips. Combining SNP microarray analysis with DNA pooling can dramatically reduce the cost of screening large numbers of SNPs on large samples, making chip technology readily applicable for genomewide association mapping in humans and farm animals. This extension, however, will require additional, nontrivial, development of FPD analytical tools.

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Tolerance to two chlorinated hydrocarbon (DDT and endrin) and two organophosphorous insecticides (trichlorfon and ronnel) in 18 Italian bee colonies was studied using topical applications. The LD50 values obtained for each insecticide were subjected to a component of variance analysis. Correlation coefficients between LD50's for the various insecticides and between LD50 and body weight and fat content were also calculated. From a consideration of the variation between colonies, it is suggested that selection based on the exploitation of existing polygenic variation can provide only minor increases in LD50's, and that development of resistant bee strains will require methods that capitalize upon major gene mutations. RÉSUMÉ VARIABILITÉ DE LA TOLÉRANCE AUX INSECTICIDES DANS DIX‐HUIT COLONIES D'ABEILLES DOMESTIQUES La tolérance envers 2 organochlorés (DDT et endrin) et envers 2 esters phosphoriques (trichlorfon et ronnel) a été étudiée dans 18 colonies d'abeilles italiennes. La méthode employée a consisté dans l'application sur la partie dorsale du thorax, d'une solution acétonique d'insecticide. Les abeilles ont été maintenues en groupes de 10 individus dans des sacs de polyvinyl à une température de 26° C et 70 H.R., avec une réserve d'eau et un mélange de sucre et de miel. La mortalité a été évaluée après 24 heures. L'action de chaque insecticide sur une colonie a été évaluée trois fois pendant l'expérience. La teneur en lipides des abeilles de chaque colonie a été déterminée par extraction à l'éther. Les valeurs der DL50 obtenues ont été analysées statistiquement. On a évalué aussi les coefficients de corrélation entre les DL50 pour chaque insecticide ainsi que la relation entre la DL50, le poids des insectes et leur teneur en lipides. La fréquence de distribution de DL50 pour chaque colonie a été normale pour le trichlorfon, le ronnel et l'endrin tandis qu'elle était irrégulière pour le DDT: les valeurs de DL50 pour 2 colonies étaient 2 fois plus élevées que pour les autres. Les différences dans les valeurs des DL50 entre les colonies étaient significatives à l'exception de l'endrin. Le degré de répétition de ces différences était très marqué pour le ronnel, moyen pour le DDT, et assez bas pour l'endrin et le trichlorfon. Une corrélation positive a été décelée entre la résistance envers certains insecticides. Cette corrélation a été plus marquée entre le trichlorfon et le ronnel, le DDT et le ronnel, moins marquée entre le DDT et l'endrin, et entre le trichlorfon et l'endrin ou le ronnel et presque nul entre le ronnel et l'endrin. On a trouvé une corrélation directe assez importante entre le poids des insectes vivants et la DL50 pour les produits organo‐phosphorés et une corrélation indirecte entre la teneur en lipides et la DL50 pour le DDT. En considérant les différences entre les colonies, on arrive à la conclusion que la sélection basée sur l'utilisation des variations polygéniques existantes peut expliquer seulement des augmentations minimes dans les valeurs de la DL50 (1 à 2 μg), mais le développement des...