How does the PITX1 pelvic enhancer mutation affect pelvic spine development?

• A. It changes pelvic coloration
• B. It increases pelvic spine development
• C. It has no effect on pelvic structures
• D. It reduces or abolishes pelvic spine development by altering gene expression in the pelvic region

Answer: (D)

Regulatory mutations in enhancers can change where and how strongly a gene is turned on, shaping the development of specific body regions. A mutation in the pelvic enhancer of PITX1 reduces its expression in the pelvic region, so the cells that would normally form the pelvic spine receive less developmental instruction. Since PITX1 is needed to drive pelvic spine formation, this weaker signal leads to reduced or abolished pelvic spine development while other parts of the body are largely unaffected. In short, altering gene expression in a targeted region via an enhancer mutation can directly change the size or presence of a structure in that region, which is why this mutation reduces pelvic spine development.