Development poses some of the central questions of biology: How does a single cell become a complex multicellular organism like us? What role do our genes play in the processes of development? From the early decades of the twentieth century, geneticists knew about mutants that altered phenotypes because of the actions of various genes during development. In numerous cases biologists knew where on the chromosome the mutant gene was located and how the mutant allele was transmitted from parent to offspring. Nevertheless, the actual role the genes play in development remained a "black box" mystery until around 1980.
Starting in the late 1970s geneticists figured out the details involved in the genetic control of development in model systems such as the fruitfly Drosophila melanogaster. They found that many of these developmental genes shared similar features. During the 1980s and 1990s geneticists made an even more surprising discovery: the same principles, and often the same genes, involved in development in model organisms (such as fruit flies and zebrafish) are also involved in controlling development in most other animals, including humans.