Each gene is made of DNA. Some of the gene's DNA carries a code or blueprint that makes a protein to carry out some specific function in an organism. Some genetic mutations change this "coding region" and thus change the protein a gene makes.
Other mutations change other parts of the gene, known as "regulatory sequences," which decide when and where the gene and its protein act in an organism's body.
Because the gene's regulatory sequences can be 10 to 100 times larger than the gene's coding region, mutations are more likely there.
A key question was whether the Hoxa1 and Hoxb1 genes are different because their protein-coding regions have changed or their regulatory sequences have changed. So the scientists switched the two genes' coding regions. Each gene then produced the other gene's protein. Mice born with the switched genes were essentially normal.
That means the coding regions were interchangeable, and that evolution has changed each gene's regulatory sequence, not the protein-making coding region.
Next, Tvrdik and Capecchi took a small portion of the regulatory sequence from gene Hoxb1 (which controls facial expressions) and put it into Hoxa1, (which allows mice to breathe and survive after birth). And they disabled the remainder of Hoxb1.
Mice suffered facial paralysis when they were born with disabled Hoxb1 and without a portion of that gene inserted into the Hoxa1 gene. In response to a puff of air in the face, they couldn't blink their eyes, wiggle their whiskers and fold back their ears.
But when a portion of the Hoxb1 regulatory sequence was inserted into Hoxa1, the new gene performed the jobs of both genes. Mice born with the combined
Source:University of Utah Health Sciences Center