Final answer:
The expression of the Antennapedia gene in an incorrect location in fruit flies is due to a homeotic mutation for a dominant mutant allele. This type of mutation alters the expression of Hox genes, leading to dramatic phenotypic changes such as the development of legs on the head instead of antennae.
Step-by-step explanation:
The Antennapedia mutation causes fruit flies to develop legs in the places on their heads where antennae would normally be found. The expression of the Antennapedia gene product in a location where it is not usually active is due to a homeotic mutation for a dominant mutant allele.
Homeotic mutations, like the Antennapedia mutation, lead to body parts forming in the wrong places. This particular type of mutation affects genes that play critical roles in body layout during embryonic development, known as Hox genes. In the case of the Antennapedia mutant, the mutation causes an expansion in the distribution of the gene product which leads to the development of legs on the head, instead of the antennae.
This anomaly underlines the power of single gene mutations to dramatically alter the phenotype. Understanding such phenomena is crucial, as insights from Drosophila genetics are widely applicable—even to human genetics, particularly in the study of genetic diseases and developmental biology.