Practice question

Proximal-distal regionalization into stylopod humerus-femur, zeugopod radius-ulna-tibia-fibula, and autopod hand-foot is governed by nested HoxA and HoxD clusters obeying spatial and temporal collinearity. Hox9 and Hox10 paralogs expressed early proximally confer stylopod identity, Hox11 marks zeugopod, Hox12-Hox13 mark autopod including carpals-tarsals and digits. Pax genes control eye and dermomyotome, Wnt governs dorsoventral axis, TGF-beta modulates growth broadly. Loss of specific Hox paralogs causes segment-specific deletions and homeotic transformations, confirming Hox code assignment of segmental identities and timing. Conserved across chick mouse human models, this mechanism illustrates classic embryological principles integrated with modern molecular genetics.