Correct Statement Regarding Shh Function in Limb Development

• Specification of digit identity primarily depends on the duration of Shh gene expression, with a secondary influence from the concentration of Shh protein that limb bud cells receive.

• This means the length of time cells are exposed to Shh signals is the major factor in defining digit identity, while the gradient concentration also modulates patterning.

• This model explains how digits form in a temporal sequence influenced by Shh’s sustained signaling rather than solely by its concentration gradient.

Why Other Statements Are Incorrect

• Shh-secreting cells do not predominantly undergo apoptosis after their signaling function.
  They primarily serve as a signaling center in the ZPA throughout limb development.

• Descendants of Shh-secreting cells do not predominantly become anterior limb bones or muscles.
  Shh-expressing cells are located in the posterior limb and contribute mainly to posterior structures.

• Simultaneous knockout of Shh and G/i3 genes does not necessarily result in complete absence of digits.
  Genetic studies have shown complex interactions where loss of one can mitigate effects of the other, and some digit development can occur.

Summary

• Shh functions as a morphogen in the limb bud, encoding positional information via both concentration gradients and duration of signaling.

• Digit specification depends more critically on how long Shh is expressed than on the exact concentration received.

• This temporal aspect of Shh signaling is crucial for orderly digit formation along the anterior-posterior axis.

Final Answer

The correct statement is:
(4) Specification of the digit is primarily dependent on the amount of time the Shh gene is expressed and to a small extent on the concentration of the Shh protein that other cells receive.