This shouldn't be a trick question. While there are some of cross-over authors that can, and sometimes are shelved in more than one place in your neighborhood bookstore, you really NEED to know where your manuscript fits into the big scheme of things. Most cross-overs, like Diana Gabaldon, are big sellers making the use of space profitable.
Why is genre-labeling so important? Think about it this way: A bookstore has a finite amount of shelf space. They want to stock it with titles that will sell, thus making that space profitable. If you're predominatly a horror reader, and you go to this section and don't find what you're looking for, you might pick up something similar. The bookstore has made a sale and you've tried a new author or a new title. If everything is all jumbled together, you may or may not find something that fits your taste. And that's bad for merchants and authors.
If you're in the process of trying to sell your first book, this is an especially important question. You want to make sure you can answer the above question without a lot of waffling. Agents and editors want to know where you fit. If you don't know what you write, and you're not able to pinpoint it in a sentence or two, the agent or editor surely can't tell you. And it's nearly impossible (if not totally impossible) to sell your book to someone who doesn't represent or acquire your genre. It's not that they don't like you or even that they're making any judgement of your work. They simply don't deal with your kind of book. Not to mention it wouldn't be in your own best interest to sign with someone who doesn't know your market.
Let me give you an example: I.M. Gonna Beanauthor has a critique appointment with Stellar Agent who represents only mystery. Beanauthor writes fantasy. The critique is a waste of time for both people. Stellar can't speak to the marketability of the book and Beanauthor gets no valueable feedback. If Beanauthor had done a little research and a little reflection on his work, he might have chosen someone better qualified to critique him. And Stellar Agent might have found an author that better fit his agency.
How do you figure out where you fit? It's simple. Read. Read widely from several genres. Figure out what the common thread is in each one. Then it's an easy question to answer. Just because two people fall in love in your book, it isn't necessarily romance. And just because there's an unsolved murder, it doesn't mean it's a mystery. Consider all the elements of your manuscript and then call it. Call it something! And be able to explain your label.
Once a week for the next few weeks, I'll focus on each major genre, one blog at a time, so that you can consider all this before you choose your faculty critique.