The puzzle: same DNA, different cells
A grown body has hundreds of cell types — neurons, muscle, skin, blood — yet almost every cell carries the same genome. So the difference between a brain cell and a liver cell is not which genes they own, but which genes they use. Developmental genetics is the study of how that selective use of genes turns one fertilized egg into a patterned, three-dimensional organism.
The key idea is that development is gene regulation in space and time. A cell reads its position in the embryo, switches on a specific set of genes through transcription factors, and so commits to becoming, say, a wing rather than a leg. Get the regulation right and you get a normal phenotype; disturb it and the body plan changes in striking, informative ways.
Four questions developmental genetics keeps asking
- How does a cell know where it is? Cells read concentration gradients of signaling molecules called morphogens to learn their position along the body axis.
- What does it do with that information? It switches on master genes — including homeotic genes — that set the identity of each body region.
- How is the right shape built? Patterned gene expression tells cells when to divide, move, change shape, or die — sculpting the body plan.
- Why do such different animals share these genes? Because the same regulatory toolkit was inherited from a common ancestor — a fly and a mouse use related master genes to build very different bodies.
Because these questions are hard to attack directly in humans, biologists turn to a small set of model organisms — the fruit fly, the nematode worm, the zebrafish, the mouse, baker's yeast, and the small weed Arabidopsis. The rest of this track follows how those favorites unlocked the genetics of building a body.