You can modify this entry for genre, for example:
<SubTree label="Genres">
<spec type="object.container.genre.musicGenre">%<genre>%</spec>
<spec type="object.container.album.musicAlbum">[$swapprefix(%album artist%) - ]%album%</spec>
</SubTree>
to this:
<SubTree label="Genres">
<spec>%<genre>%</spec>
</SubTree>
Please be aware that when selecting a genre, it will return a list of all tracks with that genre (sorted by album and ? in my tests, but you have no control on how they're listed) and that without any subnodes, [All Tracks] will not appear.
Thus modifying the subtree to this:
<SubTree label="Genres">
<spec type="object.container.genre.musicGenre">%<genre>%</spec>
</SubTree>
will have not make [All Tracks] appear because there isn't a spec for a subnode.
[All Tracks] nodes are achieved by these specs, by the way:
<spec type="object.container.person.musicArtist">
<spec type="object.container.album.musicAlbum">
<spec type="object.container.genre.musicGenre">