During the first three weeks after conception, whole batteries of genes are turned on and off in our gill arches and throughout the tissues that will become our future brain. These genes instruct cells to make the different portions of our head. Think of each region of our head as gaining a genetic address that makes it distinctive. Modify this genetic address and we can modify the kinds of structures that develop there.
For example, a gene known as Otx is active in the front region, where the first gill arch forms. Behind it, toward the back of the head, a number of so-called Hox genes are active. Each gill arch has a different complement of Hox genes active in it. With this information, we can make a map of our gill arches and the constellation of genes active in making each.
Now we can do experiments: change the genetic address of one gill arch into that of another. Take a frog embryo, turn off some genes, make the genetic signals similar in the first and second arches, and you end up with a frog that has two jaws: a mandible develops where a hyoid bone would normally be. This shows the power of the genetic addresses in making our gill arches. Change the address, and you change the structures in the arch. The power of this approach is that we can now experiment with the basic design of heads: we can manipulate the identity of the gill arches almost at will, by changing the activity of the genes inside.
Was this article helpful?