The standard prisoner's dilemma

This isn't a prisoner's dilemma. But the term 'defection' is not specific to the prisoner's dilemma.

I don't know whether unequal power (access to rewards, risk and severity of punishment) has been explored.

They have. Both with actual Prisoner's Dilemmas with non-equivalent payoffs and in various other games which take similar forms. In the former case the guy with the lower payoff tends to defect more to try to get 'equality'. But that is in a situation in which the subjects consider themselves of equal status (probably volunteer undergrads).