When the gene which functions in the posterior segments gets deleted, the immediate anterior segments extend their function in the next posterior direction. If the gene that acts in the anterior segments gets deleted, the extension of the next posterior segment in the anterior direction is not allowed.
The initial expression patterns of the homeotic genes are controlled by gap genes.
The expression of the gene Abd-A in wild-type embryos are seen in the posterior abdominal segments. The deletion of this gene leads to the expression of receive gene that is abd-A (which is expressed in the middle abdominal segments in the wild type). It is extended towards posterior direction and is expressed in the middle and posterior abdominal segments.
In the same way, when Abd-B gene is deleted, the expression of Abd-A gene is extended in posterior direction to the next posterior region that is, posterior abdominal segments.
Also, when Abd-A is deleted, Ubx (which is expressed in posterior thoracic and anterior abdominal segments in the wild type), is expressed in the posterior thoracic and anterior and middle abdominal segments. This is because, in the absence of Abd-A, Ubx expression is extended to the next posterior region towards the posterior direction that is to middle abdominal segments.
Also, when Ubx is deleted, the expression pattern of Abd-A and Abd-B expression is unchanged. The reason is that the deletion of anterior gene Ubx does not allow the extension of the next posterior segment Abd-B in the anterior direction.
The gap gene activate the UBX homeodomain gene in both thoracic and abdominal segments but Abd-A and Abd-B genes are activated only in the middle and posterior abdominal segments.
The function of Abd-A and Abd-B does not allow the expression of Ubx in middle and posterior segments. If Abd-A and Abd-B are deleted, the expression of Ubx can be extended towards the posterior direction to the middle and posterior segments.