The reasoning in this thread, so far, is sound; but i believe that some of the basics have been missed. Firstly, is it not impossible for a genre to exist without a text that adheres to its conventions and definition? Therefore, a genre can even be created by the emergence of a radical new text which defies definiton in existing genre labels.
Also, texts can 'stretch' or expand the boundaries of a genre by playing on its conventions and definitions. At this point it is important to remember that literary demand is a great influencing factor, and if a text 'stretches' a genre in an appealing way, other texts will follow suit and the seat of the genre will shift towards this new trend.
Genres are highly dynamic and fluctuating now, as opposed to their original, rigid definitions. To end I would like to quote a helpful extract from a work by Simon and Mathhew Brown (from HSC online): "There are a number of factors constantly at work on the development of generic concerns: authorial and technological experimentation, economic and social changes, readership demands, and the shifting status of individually defined genres."
I hope this has helped clarify the situation.