Shares in Genentech, the US biotech company, jumped sharply on Monday amid investor hopes that Roche, its Swiss parent, may be forced to increase a $44bn offer to buy out all minority shareholders.

The offer, which may yet be contested by Genentech’s board, marks a reversal of Roche’s policy since it bought a majority stake in 1990 to maintain Genentech’s entrepreneurial culture by keeping it at arms’ length.