Final answer:
The role of the upper house in a majoritarian system is to provide checks and balances to the lower house.
Step-by-step explanation:
The role of the upper house when a major political party controls both the lower and upper house is to provide checks and balances to the lower house.
In this situation, the upper house acts as a counterbalance to the power of the majority party in the lower house, ensuring that legislation is thoroughly examined and debated before being enacted into law.
This system helps prevent the concentration of power within one party and promotes accountability and transparency in the legislative process.