Practice question

Lmx1b executes dorsal limb program downstream of dorsal ectodermal Wnt7a. In knockout mice, dorsal mesenchyme fails to acquire dorsal characteristics: nails become absent or replaced by ventral-like skin with foot pads, dorsal extensor tendons and muscles duplicate ventral flexor patterns, producing biventral double-ventral limbs with mirror symmetry. Ventral structures remain intact because ventral pattern driven by En1 and BMP signaling independent of Lmx1b. Digit fusion does not dominate; phenotype represents clear dorsal-to-ventral transformation highlighting necessity of Lmx1b for dorsal specification and maintenance of limb asymmetry.