The agreement also gives Genius the option to buy the businesses and assets of Finsbury's UCB and Livwell bakery subsidiaries, which produce Finsbury's gluten-free products as well as yum-yums and potato scones, for £21m.
Genius has transferred the potato scone and yum-yum business back to Finsbury and these would be retained rather than sold back to the company in the event of sale.
UCB and Livwell posted combined revenues of £23.1m and pre-tax profits of £1m for the year to 30 June. Finsbury said that proceeds froom any sale would be used to repay debt and "for general corporate purposes".
"We are looking forward to further developing the strong team that has been working with Finsbury as a key partner in the UK," said Genius chief executive Gervase Cottam. "Both parties are aligned and focused on developing great-tasting gluten-free products that deliver against the exacting demands of our customers and consumers."
Revenues at Finsbury were up 5% for the 17 weeks to 31 October, buoyed by a bread and free-from division that grew by 12%.
Through the JV, the two companies hope to broaden the appeal of Genius beyond coeliac sufferers and ramp up its category share from 25% to 40% through a £3m marketing campaign.