def make_vertex_2_gene_dict(cursor):
    """ Create a dictionary that maps vertices to the genes that they belong to.
    """
    vertex_2_gene = {}
    query = """SELECT * FROM vertex LEFT JOIN genes ON vertex.gene_ID = genes.gene_ID"""
    cursor.execute(query)
    for vertex_line in cursor.fetchall():
        vertex = vertex_line["vertex_ID"]
        gene = vertex_line["gene_ID"]
        strand = vertex_line["strand"]

        if vertex in vertex_2_gene:
            vertex_2_gene[vertex].add((gene, strand))
        else:
            vertex_2_gene[vertex] = set()
            vertex_2_gene[vertex].add((gene, strand))

    return vertex_2_gene