Restriction Fragment Length polymorphisms in the human genome will be discovered using as probes single-copy DNA segments isolated from cosmids. These polymorphisms will be mapped to specific regions of human chromosomes. Multiple polymorphisms that map within single cosmid inserts and that are in linkage equilibrium will be extremely useful markers for assigning genes that cause serious genetic disease to specific regions of human chromosomes. In the near future such information can be expected to contribute to our ability to diagnose such diseases, and therefore to counsel subjects at risk. In the long run such information may lead to the isolation of defective genes and their normal counterparts, with important implications for the treatment of serious genetic diseases. Cosmids from a human genomic library will be screened using a novel blot hybridization technique to determine which ones have inserts homologous to genomic regions especially rich in polymorphisms. These cosmids will be studied in detail by subcloning into plasmid vectors, reiteration frequency screening to select single-copy probes, and genomic blotting to select probes that reveal polymorphisms. When multiple polymorphisms are found to map within a single cosmid insert, the extent of linkage disequilibrium will be determined by measuring allele and haplotype frequencies in a population of unrelated individuals. Family studies will then be used to confirm close linkage of such polymorphisms. Chromosomal locations will be determined by in situ hybridization to metaphase chromosomes.