The science of genetics began with the work of Gregor Mendel, an Austrian monk, as early as the middle of the 19th century. Mendel s experiments with selective crossbreeding of pea plants established the fundamental principles of heredity. He described certain laws of inheritance and showed that certain physical characteristics called traits, such as ower colour, seed colour, seed shape, stem length, and so on, are passed on from generation to generation some traits called recessive are masked by dominant traits and most traits are inherited independently of others. For example, pea owers are either purple or white, and intermediate colours do not show up upon cross-breeding. However, Mendel s ndings remained unnoticed till the 20th century when geneticists experimenting on fruit ies recognized his work it was shown that there are genes in chromosomes. Speci c genetic alterations could also be linked to the change of physical characteristics of the organism, that is, genotype phenotype relations could be established. However, the bacterial genetics studies were the rst to bring in the chemical nature of the gene as well as the mechanisms by which the genotype determines the phenotype. The journey began with the breakthrough in the 1940s which showed that bacteria can be a genetic tool, and since then many of the principles of genetics as well as recombinant DNA technology have been developed around bacteria. Escherichia coli was the primary focus in all these studies, and some of the genetically useful bacterial viruses called bacteriophages were also involved in elucidating genetic principles. It may be recalled that the rst genome to be sequenced completely was a bacterial genome. Although the sequencing of E. coli genome was started before, the credit of the rst genome to be sequenced goes to Haemophilus in uenzae. Understanding the principles of bacterial and bacteriophage genetics is therefore extremely important and needs to be emphasized for understanding the areas of molecular biology and recombinant DNA technology.
Importance of Bacteria and Bacteriophages Biologists use simpler model systems which are easy to grow in the laboratory and can easily be manipulated. This is because it is dif cult to conduct experiments in complex organisms like primates and it is unethical to conduct some of the experiments on human. Bacteria are relatively simple organisms. They can be both grown easily in the laboratory and also manipulated easily. They serve as a model system to understand the cellular functions and processes in more complex organisms. Bacteria are important not only as a laboratory tool, but also as an important constituent of life on earth. They play an essential role in ecology of the earth, and have a central role in nitrogen xation and carbon cycle. They are used for degrading certain chemicals and toxic products, cleaning up waste, and leaching of metals from their ores, and, thus, are industrially important. As many bacteria can thrive in extreme environments, they are the sources of thermostable enzymes. Some bacteria like symbiotic bacteria are found in humans and other organisms, and are bene cial for them. However, a number of bacteria exist which are pathogenic to human. It is necessary to identify and study these pathogenic organisms so that drugs can be developed and human beings can be protected against diseases. Bacteria, phages, and the viruses that infect bacteria are worth studying as they serve as the source of many useful enzymes. As have started understanding these organisms, one of the developments in these areas has been the availability of complete genome sequences of many bacteria and phages. However, a substantial part of the protein coding regions are of unknown function. Also