def col_frequencies(col, weights=None, gap_chars='-.'):
    """Frequencies of each residue type (totaling 1.0) in a single column."""
    counts = col_counts(col, weights, gap_chars)
    # Reduce to frequencies
    scale = 1.0 / sum(counts.values())
    return dict((aa, cnt * scale) for aa, cnt in counts.iteritems())