T W O mapping procedures have been used for the analysis of multiple allelic senes. Both of these methods are based on techniques successfully used for genes which are relatively far apart on the chromosome map. In one procedure, a particular set of genes is singled out to establish some standard points of reference. Other genes, known to segregate with the same linkage group, may be tested individually with one or more of these reference loci. The series of recombination frequencies which are obtained can then be arranged from the smallest to the largest frequency of recombination obtained with each reference point. Assume, for example, that a gene close to the telomere of a chromosome is chosen as a reference point. If two genes tested with it give different frequencies of recombination, the smaller frequency would permit the placement of its gene between the reference point and the locus of the gene used for obtaining the larger frequency of recombination. If the recombination frequencies are based on large counts, the approximate map distances of all the genes used in the investigation can be calculated. From such a tentative ordering of the map the predicted localizations can be more accurately determined. This method is particularly useful for making a rapid survey of the chromosome map if several genes are available. Since no other genetic tools are employed in this procedure, it is necessarily an indirect approach, employing as its basic assumption the well-established view that map distance can be measured by the frequency of recombination. When employed for very small distances, especially pseudoallelic systems, this method must also assume that there is no special feature of recombination that would disturb the relation between distance and recombination frequency. This method, as used here, shall be referred to as the indirect method of linkage mapping by overlapping recombination frequencies.For determining the sequential order, but not the map distance of two genes, another approach may be used. Two genes (or alleles) may be mapped with respect to one another by using one or more additional genes ("markers") readily distinguishable from the ones being investigated. In practice, two marker genes are used to identify, respectively, a locus to the left of the two genes being tested and a locus to their right. If a recombination does occur between the two middle genes, the distribution of the marker genes in the recovered recombinant indicates the proper linear sequence of all four of the genes involved. This method

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HE sex-linked lethal test provides an objective, quantitative approach to Tmutagenesis in Drosophila. For radiation mutagenesis the analysis is usually limited to a single generation, the F, progeny of irradiated PI males or females. This procedure was established by MULLER (1928), who demonstrated that * Corrected for spontaneous lethals estimated at 0.25 percent.+ Calculated by adding the number of F2 lethals corresponding to the surviving lines in the F3 test. Each surviving j: Actually infinity, but we are assuming at least one F, lethal to permit a comparison. nonlethal F, culture was sampled for one heterozygous Bar female. Each sample of one female constitutes one line.

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Teaching the introductory course is a continuous struggle to rescue an element of choice from the pressure of circumstances, but the student's lifelong view of science is in the balance.