seq = Seq.Seq(''.join(random.choice(('A', 'C', 'G', 'T'), p=((1 - mean_gc_frac) / 2, mean_gc_frac /
                                                                         2, mean_gc_frac / 2, (1 - mean_gc_frac) / 2), size=(seq_len, ))), Alphabet.DNAAlphabet())