The Hox genes are responsible for determining the anterior–posterior identity of body regions (segments) in the developing insect embryo. Different Hox genes are turned on (expressed) in different segments of the body, and in this way they determine which segments become head and which thorax, which develop legs and which antennas. One surprising thing about the Hox genes is that they usually occur in a row on the same chromosome and in the same order as the body regions that they control. For example, the fruit fly Drosophila melanogaster has eight Hox genes located on a chromosome in exactly the same order as the body regions in which they are expressed, from head to tail (see figure below; Lewis et al. 2003; Negre et al. 2005). If the eight genes were thrown randomly onto the same chromosome, what is the probability that they would line up in the same order as the segments in which they are expressed?

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Drosophila HOM-C Drosophila embryo lab Head ph Thorax Dfd Scr Abdomen Antp Ubx AbdA AbdB