The morphogen theory posits that proteins controlling traits are arranged as gradients, with different amounts of proteins activating genes to create specified physical features.

 

This theory was first put forth in the 1950s by mathematician and World War II code breaker Alan Turing and refined in the 1960s by Lewis Wolpert. It has been used to explain why a tiger has stripes, among other phenomena.

 

But some biologists have raised questions about the theory, which contends that physical features are necessarily tied to absolute concentrations of proteins within the morphogen gradient.

 

If a certain critical mass of protein is present, then a given physical feature—for example, cells that make the skin on your forehead—will appear. If less than that critical mass is present, a different structure—say, the skin that makes your eyebrows—will appear, and a boundary will be formed between the two structures.

Alternative views have suggested physical features are not necessarily the result of a specified number of proteins, but, rather, come from more complex interactions between multiple gradients that work against one another.