Not everyone realizes that there is more to writing in a genre than just the content of the book. Fantasy stories have elves and dwarves and vampire fiction obviously has vampires in it. Romance should, likewise focus primarily on the relationships within the story.
Some of those nuances include voicing in your characters—the way they speak, the way they see the world, and what motivates them. This goes far beyond “typin die-log like dis if you was writtin a no good southern hornswaggler as your heel.” In fact, don’t do that outside of a few rare exceptions. It makes reading terribly difficult for a reader if an entire character reads like that and interrupts the flow. Here’s what I mean, when you are writing fantasy you may make up words or write mythopoeicly (world-building) which means you might pepper descriptions in or drop nuances through the story so that certain terms can be intuitively gleaned by context. For my Spec Fic YA novel, Wolf of the Tesseract I studied a few lists of things to keep in mind for my writers.
The primary YA audience is more narcissistic by nature (it’s not an insult, growing teens always go through a psychological stage where their thought process asks “what does this mean to me?”) For the novel, I intentionally funneled as much through the character’s POV as possible and lingered more on emotion than normal. I used more personal pronouns than I often might and was intentional about using accessible language and limiting “silver dollar” words without obvious context.
Genre is about much more than just the thematic tropes. It is something that should be given a little forethought as you begin and edit the story. Always ask yourself how a reader will see your treatment of the text—for example, you may have written a romance novel meant for the Christian female market… if you are overly explicit or use profanity you can expect to turn off a huge segment of the readers (and expect bad reviews). Likewise, if you write a middle grade adventure but dwell too heavily on high prose as the child protagonist ponders psychological themes and motivations of the antagonists and world which intersects his own you will lose the reader.
Literary fiction usually references “high concept” writing. It may have elements of genre stories. Think of the movie Gravity, for example (which was a script stolen by legal loopholes from novelist Tess Gerritsen). It involved astronauts and action in space, but the main point of the story was not the space-walk or an adventure—it was the theme of an invisible force calling us to overcome adversity and cling to life. That’s a high-concept example making it literary.
Outside of “literary fiction” many individual thematic genres exist. Each one will have its own set of rules (expected word count, appropriate styles and language, etc.) and should be researched. These include labels such as paranormal, horror, science fiction, fantasy, romance, western, nonfiction, commercial, crime, and so many more.)
Age genres will sometimes stack with thematic tags but help specify the type of target audience. YA novels are typically for teenagers while MG (middle grade) feature stories written from 8-12 year olds. NA (new adult) is target readers around college age and is a fairly new bullseye recognized by the market; NA is primarily meant for 18-24 year olds.
Like all things, do your research so you can write the best book possible, and write to the rules rather than the exception for true success!