HOX gene expression along the craniocaudal axis of the embryo follows the colinearity principle. Which statement correctly describes this principle?

• A Genes located at the 3' end of the HOX cluster are expressed more anteriorly and earlier in development than 5' genes ✓
• B Genes located at the 5' end of the HOX cluster are expressed more anteriorly and earlier in development
• C All HOX genes are expressed simultaneously but in different concentrations
• D HOX genes show no spatial colinearity, only temporal colinearity

Explanation

Spatial and temporal colinearity is a fundamental property of HOX genes: genes at the 3' end of the cluster (e.g., HOXA1, HOXB1) are activated earlier in development and specify anterior (more cranial) structures, while genes at the 5' end (e.g., HOXA13, HOXD13) are activated later and specify posterior (more caudal/distal) structures. This reflects the physical order of genes on the chromosome mirroring their expression domains along the body axis. Mutations in posterior HOX genes (e.g., HOXD13) cause synpolydactyly and hand-foot-genital syndrome.

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

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Written and medically reviewed by the StethoPrep medical team.