The three homeodomain proteins ABD-B, ABD-A, and UBX are encoded by genes within the BX-C of Drosophila. In wild-type embryos, the Abd-B gene is expressed in the posterior abdominal segments, abd-A in the middle abdominal segments, and Ubx in the anterior abdominal and posterior thoracic segments. When the Abd-B gene is deleted, abd-A is expressed in both the middle and the posterior abdominal segments. When abd-A is deleted, Ubx is expressed in the posterior thorax and in the anterior and middle abdominal segments. When Ubx is deleted, the patterns of abd-A and Abd-B expression are unchanged from wild-type. When both abd-A and Abd-B are deleted, Ubx is expressed in all segments from the posterior thorax to the posterior end of the embryo. Explain these observations, taking into consideration the fact that the gap genes control the initial expression patterns of the homeotic genes.