[Campbell Biology P.1112] The text states that Sonic hedgehog is the protein signal secreted ... | Practice Question

The text states that Sonic hedgehog is the protein signal secreted by the ZPA. Considering the "WHAT IF?" scenario that the ZPA forms after the AER, and the AER is necessary for ZPA formation, how would removing the AER and then checking for Sonic hedgehog expression test this hypothesis?

  • A: If the hypothesis is true, Sonic hedgehog expression would increase significantly throughout the limb bud, confirming AER's regulatory role.
  • B: If the hypothesis is true, the absence or significant reduction of Sonic hedgehog expression in the limb bud after AER removal would support the hypothesis.
  • C: Sonic hedgehog expression would remain unchanged, as the AER and ZPA function independently in limb patterning.
  • D: Removing the AER would cause Sonic hedgehog to be expressed in an ectopic anterior location, leading to abnormal limb development.

Explanation

The "WHAT IF?" scenario proposes that if AER is necessary for ZPA formation, then removing AER should prevent ZPA from forming. Since Sonic hedgehog is the signal secreted by ZPA, if ZPA doesn't form, then Sonic hedgehog expression should be absent or significantly reduced. Therefore, observing the absence or reduction of Sonic hedgehog expression after AER removal would directly support the hypothesis that AER is necessary for ZPA formation.