HOX gene clusters A–D are expressed along the craniocaudal axis. Which principle best explains the phenotypic consequence of mutations in posterior HOX genes?

• A Posterior transformation: posterior segments adopt more anterior identities
• B Anterior transformation: anterior segments adopt more posterior identities
• C Posterior transformation: anterior segments adopt more posterior identities ✓
• D No phenotypic change due to functional redundancy

Explanation

The Hox gene collinearity principle (posterior prevalence) dictates that more posterior Hox genes repress more anterior ones. Loss-of-function of a posterior Hox gene causes anterior segments to adopt more posterior identities because the repressor is absent — this is called posterior transformation (anterior structures gain posterior character). Gain-of-function causes anterior transformation. This principle explains homeotic mutations such as cervical rib formation when HOXA genes are dysregulated.

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

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