HOX gene clusters provide positional identity along the cranio-caudal axis. Loss-of-function mutations in HOX genes typically produce:

• A Homeotic transformations where a posterior segment acquires a more anterior identity (posterior → anterior)
• B Homeotic transformations where an anterior segment acquires a more posterior identity (anterior → posterior) ✓
• C Segmental duplication of the entire axial skeleton
• D Absence of all somite differentiation along the neural tube

Explanation

HOX genes follow the principle of 'posterior prevalence' — more caudally expressed HOX genes suppress the action of more cranially expressed ones. Loss-of-function mutations remove this posterior suppression, resulting in an anterior homeotic transformation where a posterior segment adopts the morphological identity of a more anterior segment (e.g., lumbar vertebra resembling a thoracic vertebra). Gain-of-function mutations cause the reverse (posterior transformation). This concept is fundamental in understanding vertebral and limb identity specification.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.