def get_gap_symbol(seq):
    """Return gap symbol.

    seq: Sequence object or plain string.

    Should be able to handle cogent.core Sequence and ModelSequence object.
    If the input sequence doesn't have a MolType, '-' will be returned
    as default.
    """
    try:
        gap = seq.MolType.Alphabet.Gap
    except AttributeError:
        gap = '-'
    return gap