Practice question

Proximal-distal patterning follows temporal colinearity of HoxA and HoxD clusters. Early-expressed Hox9 paralogues mark stylopod progenitors forming humerus and femur, intermediate Hox10-11 specify zeugopod radius ulna tibia fibula, while late 5-prime genes Hox12 and Hox13 are activated distally under prolonged FGF exposure and repressed proximally by retinoic acid and Meis factors. Combined Hoxa13 and Hoxd13 activity is essential for autopod wrist ankle and digits. Deletion abolishes digits, whereas Hox9 or Hox11 mutants affect proximal segments, definitively defining autopod genetic code.