Drosophila melanogaster has four pairs of chromosomes and a compact genome, published in 2000, containing roughly fourteen thousand protein-coding genes. Comparisons made after that sequence appeared found that a large majority of genes implicated in human disease have an identifiable fly homologue, most often cited as around three quarters. The figure depends on the disease-gene list used and on how loosely homology is defined, so it should be read as an order of magnitude rather than a constant.
The practical advantages are as important as the genetics. A generation takes about ten days at 25 degrees Celsius, hundreds of offspring can come from a single pair, and the salivary glands of larvae contain giant polytene chromosomes whose banding can be read under a light microscope.
Work on the fly underpins several Nobel Prizes, including Thomas Hunt Morgan in 1933 for chromosomal heredity, the 1995 prize for early embryonic patterning, and the 2017 prize for the molecular clock that governs circadian rhythm.