Final answer:
The spineless aristapedia (ssa) mutation in Drosophila affects a homeotic gene, resulting in the development of leg parts where antennae should be. Homeotic genes with homeoboxes dictate the body plan and placement of appendages, making them vital for proper organism development.
Step-by-step explanation:
In Drosophila, the developmental mutation called ssa (spineless aristapedia) that results in flies with mini leg parts on the antennae affects a type of gene known as a homeotic gene. Homeotic genes contain DNA sequences called homeoboxes, which encode protein transcription factors responsible for determining the general body plan of an organism, including the number and placement of appendages. Mutations in a Drosophila Hox gene, which is a type of homeotic gene, can lead to transformative phenotypic changes, like legs growing in place of antennae, since both antennae and legs are embryologic homologous structures.
These Hox genes play a crucial role in the anterior to posterior body plan of an organism, making sure that each body part develops in the correct location. The uniqueness of the mutation caused by the spineless aristapedia gene underscores the importance of homeotic genes in maintaining the usual body plan and preventing large scale, asymmetrical changes such as appendages emerging from atypical locations.