The last common ancestor of all bilaterians is thought to have had four Hox genes. Most extant cnidarians have two Hox genes, though some have three Hox genes. On the basis of these observations, some have proposed that the ancestral cnidarian's were originally bilateral and, in stages, lost Hox genes from their genomes. If true, this would mean that .
A) all radially symmetric animals should be grouped together in one clade
B) the radial symmetry of extant cnidarians is secondarily derived, rather than being an ancestral trait
C) Hox genes play little actual role in coding for an animal's "body plan"
D) cnidaria may someday replace porifera as the basal bilaterians

The Hox genes, a subset of transcription factor genes, exhibit the odd trait listed below: They give us a glimpse of one mechanism by which the numerous animal (metazoan) morphologies are produced as a result of gene expression.
The genome appears to be primarily an unorganized tangle of many genes scattered about randomly on a chromosome; the pattern is only visible in their expression during development.
The Hox genes, on the other hand, seem to constitute an island of comprehensible arrangement. These genes control how segments are classified, for example, whether a specific embryonic segment will form the head, thorax, or abdomen. They are all gathered in one, typically organized location.

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