- A Hox genes are sometimes thought to function specifically in controlling flower colour pathways in plants, a role largely unrelated to their actual function specifying body segment identity in animals
- B Hox genes are homeotic transcription factors specifying body segment identity along the anterior-posterior axis in all bilateral animals; their conservation across phyla reveals deep homology
- C Hox genes are sometimes thought to be present mainly within vertebrate lineages, absent from invertebrates, despite being conserved and active across invertebrate phyla from flies to flatworms
- D Hox genes are sometimes thought to have been largely lost during evolution across most animal phyla, when in fact they remain remarkably conserved in sequence and expression across bilaterian phyla
Correct answer: B. Hox genes are homeotic transcription factors specifying body segment identity along the anterior-posterior axis in all bilateral animals; their conservation across phyla reveals deep homology
Explanation: Hox genes encode homeodomain transcription factors. Key features: (1) Conserved across bilaterians (fly Antennapedia and human HOXA9 can partially substitute for each other); (2) Colinear: Hox gene order on chromosome corresponds to anterior-posterior expression in body; (3) Expanded in vertebrates (4 Hox clusters vs 1 in fly) allowing greater morphological complexity. Mutations cause homeotic transformations (fly antennae develop as legs). Hox conservation demonstrates all bilaterians share a ~600 mya common ancestor.
Concept context
Origin of life, Darwin's theory, natural selection, speciation, and evidence for evolution.