Swift coding practice: how could I improve on this code?

// Movie class

class Movie {
    let title: String
    let genres: [Genre]
    // let poster: UIImage
    // let discription: String
    // let rating: String
    // let runtime: String
    // let releaseDate: String

    init(title: String, genres: [Genre]) {
        self.title = title
        self.genres = genres
    }

    func addToGenres() {
        for genre in genres {
            genre.movies.append(self)
        }
    }
}

// Genre class

class Genre {
    let title: String
    var movies: [Movie]

    init(title: String, movies: [Movie]) {
        self.title = title
        self.movies = movies
    }
}

// Genres

let drama = Genre(title: "Drama", movies: [])
let thriller = Genre(title: "Thriller", movies: [])
let fantasy = Genre(title: "Fantasy", movies: [])
let scienceFiction = Genre(title: "Science Fiction", movies: [])

func createGenresArray() -> [Genre] {
    var genres = [Genre]()
    genres.append(drama)
    genres.append(thriller)
    genres.append(fantasy)
    genres.append(scienceFiction)
    return genres
}

let genres = createGenresArray()

// Movies

let theShapeOfWater = Movie(title: "The Shape of Water", genres: [drama, thriller])
let wonderWoman = Movie(title: "Wonder Woman", genres: [fantasy, scienceFiction])
let starWarsTheLastJedi = Movie(title: "Star Wars: The Last Jedi", genres: [fantasy, scienceFiction])

theShapeOfWater.addToGenres()
wonderWoman.addToGenres()
starWarsTheLastJedi.addToGenres()

func createMoviesArray() -> [Movie] {
    var movies = [Movie]()
    movies.append(theShapeOfWater)
    movies.append(wonderWoman)
    movies.append(starWarsTheLastJedi)
    return movies
}

let movies = createMoviesArray()

// Testing

print("Found \(drama.movies.count) drama movies")
print("Found \(thriller.movies.count) thriller movies")
print("Found \(fantasy.movies.count) fantasy movies")
print("Found \(scienceFiction.movies.count) science fiction movies")
print("Total genres: \(genres.count)")
print("Total movies: \(movies.count)")