A contemporary problem - finding the function of genes
Genes were originally detected by the heritable phenotype generated by their mutant alleles, such as the white eyes in the normally red-eyed Drosophilaor the sickle cell form of hemoglobin (HbS) in humans. Now that we have the ability to isolate virtually any, and perhaps all, segments of DNA from the genome of an organism, the issue arises as to which of those segments are genes, and what is the function of those genes. (The genomeis all the DNA in the chromosomes of an organism.) Earlier geneticists knew what the function of the genes were that they were studying (at least in terms of some macroscopic phenotype), even when they had no idea what the nature of the genetic material was. Now molecular biologists are confronted with the opposite problem - we can find and study lots of DNA, but which regions are functions? Many computational approaches are being developed to guide in this analysis, but eventually we come back to that classical definition, i.e. that appropriate mutations in any functional gene should generate a detectable phenotype. The approach of biochemically making mutations in DNA in the laboratory and then testing for the effects in living cells or whole organisms is called "reverse genetics."