The common fruit fly, Drosophila melanogaster, is a small insect central to the history and practice of modern genetics. Its physical and genetic maps provide an accessible framework for understanding how genes are organized and inherited in multicellular organisms. The unique structure and visibility of its chromosomes offer a clear window into the relationship between a gene’s physical location and its resulting biological function.
Why the Fruit Fly is the Geneticist’s Favorite
The fruit fly’s popularity as a model organism stems from its practical biological advantages for laboratory study. Its life cycle is remarkably short, taking only about ten days from egg to mature adult. This rapid generation time allows researchers to study multiple generations and observe inheritance patterns quickly. Female flies are highly productive, laying around 100 eggs per day, which provides a large number of offspring for genetic analysis. Furthermore, Drosophila is easily and inexpensively maintained in the laboratory, requiring minimal space and simple culture media.
The Standard Chromosomal Set
The typical somatic cell of Drosophila melanogaster contains a small and simple diploid chromosomal set of four homologous pairs (2n=8). These four pairs are easily distinguishable by size and shape. Three pairs are autosomes, designated Chromosomes 2, 3, and 4. Chromosomes 2 and 3 are relatively large, while Chromosome 4 is exceptionally small, often appearing as a tiny dot. The fourth pair consists of the sex chromosomes: two X chromosomes (XX) for females or an X and a Y chromosome (XY) for males. Sex determination is based on the ratio of X chromosomes to the autosome sets, not simply the presence of the Y chromosome.
Giant Polytene Chromosomes
A unique feature benefiting genetic study is the presence of giant polytene chromosomes in certain larval tissues, most notably the salivary glands. These massive structures form through repeated rounds of DNA replication without subsequent cell division, a process called endoreduplication. This creates a single, thick cable-like structure composed of hundreds of identical, side-by-side DNA strands. Polytene chromosomes can be up to 200 times larger than standard metaphase chromosomes. This immense size makes them visible under a low-power microscope, which is a major advantage for cytological analysis. The chromosomes exhibit a characteristic pattern of alternating dark bands and light interbands along their length.
Mapping Genes Using Banding Patterns
The distinctive banding pattern of polytene chromosomes serves as a high-resolution physical map of the Drosophila genome. The dense, dark bands are composed of highly compacted, often transcriptionally inactive, chromatin. Conversely, the lighter interbands are areas of decompacted, or “open,” chromatin often associated with active gene expression. Geneticists use a standardized nomenclature to label specific locations on this physical map, dividing the chromosomes into numbered and lettered regions (e.g., “3B1”). This cytological mapping allows researchers to precisely correlate a gene’s physical address with a specific genetic trait or mutation. Scientists can visualize physical alterations like deletions, duplications, or inversions and directly link them to changes in the fly’s phenotype.