Final answer:
The ectopic expression of the Antennapedia gene in Drosophila's head results in the replacement of antennae with an extra pair of legs, indicating a homeotic mutation.
Step-by-step explanation:
The Antennapedia gene is one of the Hox genes that is involved in the development of the fruit fly (Drosophila melanogaster). When a mutation occurs in this gene, it can cause an ectopic expression, which leads to developmental anomalies. Specifically, the ectopic expression of the Antennapedia gene in the head of Drosophila results in the transformation of the antennae into legs. This means that instead of developing normal antennae on the head, the fruit fly develops an additional pair of legs. This phenotype is a result of a homeotic mutation, where one body part is replaced with another that is not typical for that location.
Therefore, the answer to the student's question is that the effect of ectopic expression of the Antennapedia gene in the head of Drosophila leads to a dramatic change where the antennae are replaced by legs. In summary, indeed the Antennapedia mutation causes an abnormal development of the head, where legs grow in the place of the head's antennae.