Mutations are beneficial if the fitness of an individual is enhanced. Harmful mutations can either slightly alter a protein, where the protein may still minimally function, or they may totally disable a protein. Since we have determined how genes can trigger harmful effects in the body, we can now examine how scientists look for specific genes that cause diseases. Scientists, looking for a diseased gene, study DNA samples in individuals where the disease has been present in the family for many generations. They look for specific genetic markers, which are DNA segments that are identified in individuals with the disease, and not identified in healthy individuals.
Scientists then narrow down the area of DNA by mapping a gene.
If a disease gene is mapped to an area, then the genelike sequences become candidate genes for a specific disease gene. If scientists can determine the map position of a known disease gene, than the unassigned genes in that region may be candidate genes. Once candidate genes are selected, scientists then look for mutations in those genes. They can identify these specific regions by constructing DNA probes, which are single strands of DNA that line up with parts of a known gene. The probe, tagged with a radioactive atom, binds to the complementary bases.