Colinearity
Rather surprisingly, it has been found that the sequence of homeotic genes in fruit flies, known as the Hox genes, are lined up in exactly the same order as the part of the fly they affect. That is to say, the first gene affects the mouth, the second the face, the third the top of the head and so on up until the eighth and final gene that affects the abdomen.
Read more about this topic: Homeotic Gene