Reverse genetics and genes that control development : initial molecular characterization of two deletion complexes in mouse chromosome 7 by random-clone mapping and genetic linkage analysis

The observation of a mutant phenotype gives insight into the normal functions associated with the gene (or genes) that has undergone mutation. The strategy of "reverse genetics," in which a gene is cloned by beginning with a phenotype that has been mapped to a known chromosomal subregion, is one way of analyzing genes that control development. Induced deletion mutations, selected by the visible mutant phenotype of a marker locus, often also cause embryonic lethality in homozygotes because other genes contiguous with the selected marker locus are deleted. Because the map position of the selected marker locus is known, the chromosomal subregion associated with the embryonic-lethality phenotype is automatically also known. If molecular access to that chromosomal subregion is achieved, then the genes responsible for the lethal phenotypes can be analyzed by reverse genetics. Two large complexes of overlapping, radiation-induced mutations of the p (pink-eyed dilution) and c (albino) loci in mouse chromosome 7, both of which are known to include recessive-lethal genes, offer the opportunity to apply a reverse-genetics strategy to the cloning of genes that control development in the mouse. Molecular access has been achieved to both regions by the mapping of random clones from genomic libraries enriched for chromosome-7 sequences. These clones have: (1) provided a point of molecular access to the distal end of the c region, near the neurological mutation, shaker-1, and near four embryonic-lethal functions: (2) provided molecular access to the p function and to closely-linked phenotypes; and, (3) provided proof that many of the radiation-induced p mutations are indeed deletions. In addition, genetic linkage analyses have determined or refined the map locations of several genes thought to lie near p. Knowledge of the relationships among the many radiation-induced p mutations has been incomplete because of a dearth of flanking-marker loci and also because of very complicated results from complementation testing of the p-lethal mutations. The mapping of these genes near p, along with the molecular probes derived from random-clone mapping, should contribute greatly to the clarification of the linkage relationships, and eventually, the functional characteristics, of p-region loci.