Final answer:
Ectopic expression of the Antennapedia gene in fruit flies can be caused by the misregulation of homeotic genes, resulting in the development of legs in place of antennae, indicating their role as master control genes conserved across species.
Step-by-step explanation:
The ectopic expression of the Antennapedia gene can be caused by misregulation of homeotic genes, which are a group of genes that determine the identity of body parts during an organism's development. In fruit flies (Drosophila), a mutation in a homeotic gene can result in the ectopic expression of the Antennapedia gene leading to the development of legs where antennae should exist, as observed in the Antennapedia mutant. This type of mutation demonstrates how homeotic genes, including the Antennapedia gene, function as master control genes influencing the development of various body segments. Their conserved nature across species shows a shared evolutionary ancestry because these genes regulate vital developmental processes in diverse organisms, underpinning the fundamental biological principle of the conservation of developmental mechanisms.