Limit category choice in entry to 1 and select child category

Question

I have a section of book entries with a categories field called "Genre". A book can fall into multiple genres. A stripped down version of the categories works something like this:

• Fiction
  • Crime, Thrillers & Mystery
    • Mystery
    • Thrillers
    • Action & Adventure
  • Fantasy
• Non-Fiction
  • etc

On the home page, there is to be a feature panel linking to a particular genre, let's say Thrillers for example.

For the feature panel field, I need to limit this to a single genre, but if I set the limit on the categories field to 1 and select Thrillers, it just selects Fiction.

Of course I could just remove the limit and then use twig to get only entries from the deepest selected category, but I don't want to open up the possibility that they'll select multiple categories.

Is there any way around this? I can't help but feel like the limit option on the categories fieldtype is a bit broken.

* Update for clarity *

Categories group called Genre is structured as above
Books channel has a category field called Genre, allowed to select multiple genres
Homepage has a category field called Related Genre

The intention is to choose a single category for Related Genre to output the books which have that category selected in Genre.

This is all entirely within the admin area, so code is effectively irrelevant at this point.

Perhaps some images will make this clearer:

In the entry, I attempt to select Thrillers:

But after clicking select, it has just selected Fiction:

Answer 1

You could use a structure for Genre rather than categories. This would let you select Thrillers as a single entry.