One of the early steps in the investigation of biological problems is to determine roughly the complexity of the system by genetic experiments. Mutations affecting the system are isolated, their effects are determined, and in some cases the mutations are mapped. Earlier we considered these questions in the abstract. Here we shall examine experimentally how these questions may be handled in bacteria, yeast, and the fruit fly Drosophila melanogaster. The same basic operations used with bacteria or yeast are also used with most other unicellular organisms or with cell cultures from multicellular organisms. Similarly, the principles and genetic operations used with Drosophila are similar to many used with other higher organisms, although the Drosophila genetic system is much more tractable than other systems.
Phage and bacteria have been important in molecular biology for many reasons. Among them is the fact that with these materials genetic experiments can easily be performed, and, most importantly, mutants can be grown and their altered genes or gene products can be isolated and definitively tested in biochemical experiments.
Yeast possess many of the virtues of bacteria. As a simple eukaryote however, many of the important questions being studied with yeast involve components or processes which are not found in bacteria or phage, for example, properties of mitochondria or messenger RNA splicing. One of the most useful properties of this organism is the ease of generating haploids and diploids. The facile generation of mutants requires use of the haploid form, but complementation studies and genetic mapping require forming diploids.
The fruit fly Drosophila melanogaster has been energetically studied by geneticists since about 1910. It is a eukaryote with differentiated tissues and is relatively easy and inexpensive to study. A large number of mutations as well as a wide variety of chromosome aberrations such as inversions, substitutions, and deletions have been catalogued and mapped in this organism. Fortunately Drosophila permits the study of many of these rearrangements with the light microscope as the chro-mosomes in the salivary glands are highly polytene. They contain about 1,000 parallel identical copies. This large amount of DNA and associated macromolecules generates banding patterns characteristic of each re-gion of the chromosome. The mechanisms of tissue-specific gene ex-pression and growth and operation of the nervous system are questions appropriate for this organism. Remarkably powerful genetics tools have been developed for investigating these questions in Drosophila. On the other hand, biochemical approaches to these questions are just begin-ning to be developed, as will be described in following.