Drosophila
Drosophila melanogaster, the common fruit fly, has been a workhorse of genetics for more than a century. A tiny fly with a ten-day life cycle, hundreds of offspring per female, and a knack for surviving in a milk bottle on mashed banana, it lets researchers track inheritance across many generations in a short time and on a small budget.
Early fly work established foundational ideas: that genes sit at specific places on chromosomes, that genes near each other tend to be inherited together, and that crossing over reshuffles them. Its large salivary-gland chromosomes are visible under a light microscope, and a rich toolkit lets scientists switch genes on and off in chosen cells and watch the consequences.
Because the fly's developmental genes have close human relatives, flies have become a premier system for studying how bodies are built and how nerves form, as well as a living test bed for diseases of the brain and other organs. Many concepts in this field — segmentation, homeotic transformation, signaling pathways — were first worked out in the fly.
A genome-wide fly screen for embryos that fail to form proper segments identified the core set of patterning genes — a study later honored with a Nobel Prize.
The fly turned the building of a body into a tractable genetic problem.
Fruit flies are diploid with just four pairs of chromosomes, and males show no crossing over — quirks that made early gene-mapping experiments unusually clean to interpret.