 Some Mendelian researchers extended the investigation of the material basis for genes to their role in developmental processes. For example, the eyes of fruit flies, normally red, are sometimes white. Geneticists identified the location on the chromosomes that corresponds to the white-eye mutation (Morgan 1919) and later investigated the pigment-formation metabolic pathway and the enzymes (proteins that modulate biochemical interactions) involved as fruit fly eyes develop the normal or mutant color (e.g., Beadle &amp; Ephrussi 1936). Research since World War II that came to be known as molecular genetics or molecular biology went on to identify DNA as the chemical basis of genes and the mechanisms of DNA replication, mutation, transcription to RNA, and translation to polypeptides (components of proteins). Researchers probed the feedback networks that regulate these mechanisms, first in viruses and bacteria, then in complex, multicellular organisms; mapped and modified the specific DNA sequence of organisms; compared sequences among taxonomic groups (i.e., groups in different branches of the classification of life) so as to assess the degree of genetic variation in populations and to classify taxonomic groups into phylogenies; traced where and when in development specific genes are active; and examined the role of DNA sequences not associated with genes (Griffiths &amp; Stotz 2013). Such research, which now occupies the center of biology, renders it plausible to many researchers and commentators that development of traits will eventually be understood in terms of a composite of the influences on the organism over time of identified DNA variants (see entry on  gene).