The Antennapedia mutation in Drosophila causes legs to grow where antennae should be. The legs are perfectly formed. What does this reveal about Hox gene function?
AHox genes directly encode the structural components of specific appendages like legs or antennae
BHox genes specify segment identity — the leg-building program is activated wherever it receives the 'leg' Hox signal, regardless of location
CThe genes for building legs and antennae are encoded within the Hox cluster and are activated by proximity
DHox mutations are generally lethal, so Antennapedia demonstrates a rare gain-of-function rescue
The key insight is that Hox genes are address labels, not structural blueprints. The leg-building developmental program is complete and intact in the antenna-forming cells — Antennapedia just delivers the wrong 'address,' instructing those cells to build a leg instead of an antenna. The legs are normal because all the leg-building machinery is present everywhere; what changed is only the Hox signal telling the cells what to build. This demonstrates that major morphological changes (antennae → legs) can result from misexpressing existing programs in the wrong location, without any changes to the structural genes themselves.
Question 2 Multiple Choice
Snakes have hundreds of rib-bearing vertebrae, while mammals typically have only a dozen or so. Comparative genomics reveals that snakes and mammals share nearly identical Hox gene sequences. This body plan difference most likely arose from:
ADuplication of Hox genes unique to the snake lineage, producing extra copies that specify extra segments
BMutations in Hox protein sequences that altered their DNA-binding specificity to activate more vertebral segments
CChanges in the regulatory control of Hox gene expression, extending the thoracic Hox domain across more segments
DLoss of the Hox genes that would otherwise suppress rib formation in lumbar and pelvic segments
This is the central evo-devo principle: body plan diversity arises largely from changes in *when and where* Hox genes are expressed (regulatory evolution), not from changes in the Hox proteins themselves. Snakes have extended the expression domain of the Hox genes that specify thoracic (rib-bearing) identity over a much larger portion of the body axis. The Hox genes themselves are conserved; it's the regulatory switches controlling them that changed. This is why the same toolkit of transcription factors can produce such radically different body plans across the animal kingdom.
Question 3 True / False
Hox gene collinearity refers to the correspondence between the position of a Hox gene on the chromosome and the body region along the anterior-posterior axis where that gene is expressed.
TTrue
FFalse
Answer: True
Collinearity is one of the most striking features of the Hox system: the gene at the 3' end of the cluster is expressed in the most anterior body region (head), and successively more 5' genes are expressed in successively more posterior regions, down to the tail. This spatial correspondence between chromosome order and body axis order is conserved from flies to humans, providing compelling evidence of a shared ancestral Hox cluster. The conservation of collinearity across ~700 million years of evolution suggests that the chromosome organization of the Hox cluster is functionally important for its sequential activation.
Question 4 True / False
Because Hox genes are conserved across essentially most animals, differences in Hox protein sequences are the primary driver of body plan diversity between species.
TTrue
FFalse
Answer: False
This is the key misconception in evo-devo. Hox protein sequences are highly conserved — a fly Hox gene introduced into a mouse can function in the correct context. Body plan diversity arises primarily from differences in the *regulatory control* of Hox genes: which cells express which Hox genes, at what levels, and when. Changes to cis-regulatory elements (enhancers, promoters) that control Hox gene expression can produce dramatic morphological differences with minimal changes to the protein-coding sequences. Evolution 'tinkers with the regulatory switches controlling the ancient toolkit it already has.'
Question 5 Short Answer
Why do evolutionary biologists describe Hox genes as an 'address system' rather than a 'blueprint'? What does this distinction reveal about how body plan evolution works?
Think about your answer, then reveal below.
Model answer: A blueprint specifies the actual structure to be built; an address system says 'this is location X' and delegates all structural decisions to downstream programs. Hox genes do the latter: they specify segment identity (anterior vs posterior, head vs abdomen) but do not directly encode the structures themselves. The leg-building genes, eye-building genes, and wing-building genes are separate programs activated by the Hox 'address.' This means body plan evolution doesn't require inventing new structural genes — it requires only changing where and when the ancient Hox addresses are assigned. Regulatory changes in Hox expression deploy existing structural programs in new configurations, producing major morphological innovation from relatively small genetic changes.
This distinction is central to the evo-devo field. Because Hox genes are addressing machinery rather than building machinery, the same toolkit can generate enormously diverse body plans by varying the addressing. It also explains why Hox genes are so conserved even across species with radically different body plans — the addresses are kept, but the territory each address covers changes.