HOX gene expression along the craniocaudal axis follows collinearity. Which statement best describes spatial collinearity of HOX genes?

• A Genes at the 3' end of the cluster are expressed more anteriorly; genes at the 5' end are expressed more posteriorly ✓
• B Genes at the 5' end are expressed more anteriorly; genes at the 3' end are expressed more posteriorly
• C HOX genes are expressed uniformly along the entire body axis without spatial restriction
• D Each HOX gene is expressed only in a single somite and no rostral–caudal gradient exists

Explanation

Spatial collinearity is a fundamental rule: HOX genes located at the 3' end of each cluster are expressed in more anterior (rostral) body regions, while those at the 5' end are expressed more posteriorly. For example, HOXA1 (3' end) is expressed in rhombomere 4 of the hindbrain, while HOXA13 (5' end) is expressed in the distal limb and cloaca. This same principle applies to temporal collinearity: 3' genes are expressed earlier in development. Mutations shifting HOX expression boundaries cause homeotic transformations (e.g., cervical rib due to altered thoracic–cervical boundary).

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

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