http://www.fao.org/docrep/006/y4011e/y4011e0v.htm
Quite an "obscure" topic but the internet is amazing!!
Essentially, if there is a new type, it MUST be bred PRECISELY to insure that a certain characteristic stays. Wheat plants are NOT clones (unlike what I thought), so they CAN lose "vitality" if they are not carefully raised in a certain fashion for various generations.
I have learnt quite a lot about wheat recently and noe have a much clearer idea of what REALLY happened with the Miracle Wheat scenario.