This is an excellent question with no easy answer. Drosophila is one model organism for which we know something about how expression of specific genes leads to

This is an excellent question with no easy answer. Drosophila is one model organism for which we know something about how expression of specific genes leads to developmental structure [1] but there is a lot still unknown, not just for Drosophila but also for cellular differentiation [2] more generally. [1]: https://en.wikipedia.org/wiki/Drosophila_embryogenesis https://en.wikipedia.org/wiki/Drosophila_embryogenesis [2]: https://en.wikipedia.org/wiki/Cellular_differentiation https://en.wikipedia.org/wiki/Cellular_differentiation