In Pitx1 knockout mice, what is observed about hind limbs and pelvic structures?

• A. Hind limbs fail to develop completely; only remnants of the pelvis and hind limbs remain, with the left side larger.
• B. Complete hind limbs develop normally.
• C. No pelvis forms, and hind limbs are absent.
• D. The pelvis and hind limbs are fully formed but asymmetrical with the right side larger.

Answer: (A)

This question tests how a gene controls hindlimb formation. Pitx1 is a transcription factor that drives hindlimb identity and outgrowth. When Pitx1 is knocked out, hindlimb development is severely impaired: the limb buds don’t grow properly, and you end up with only vestigial remnants rather than fully formed hindlimbs. Pelvic structures can still form to some extent, so you may see remnants of the pelvis alongside the tiny limb tissue, and developmental processes can produce asymmetry between left and right sides.

So the described observation—hindlimbs largely failing to develop with residual pelvic and hindlimb tissue, and one side sometimes larger than the other—fits what Pitx1 loss would produce. The other scenarios describe normal hindlimb development, complete absence of pelvis, or fully formed pelvis and limbs with asymmetry, which don’t align with the consequences of knocking out Pitx1.