British Columbia, Canada
The following excerpt is from Wood v. Porter, 2015 BCSC 2354 (CanLII):
The presumption of undue influence arises in circumstances where the relationship between the parties gives rise to the potential domination of one party by another. Once a dominant relationship has been established such that potential for influence exists, the onus moves to the defendant to rebut it and show that the plaintiff entered into the transaction as a result of his own “free, full and informed thought”: Geffen v. Goodman Estate, 1991 CanLII 69 (SCC),  2 S.C.R. 353 at paras. 42-45.