def hamming(seq1, seq2):
    """ Returns hamming distance between two sequences
    
    Parameters
    ----------
    seq1 : query sequence
    seq2 : reference sequence
    
    Returns
    -------
    integer hamming distance between query sequence and reference sequence
    
    """
    return sum(1 for ch1, ch2 in zip(seq1, seq2) if ch1 != ch2)