def _get_relationships(notation):
    """
    Return the list of orthology relationships to keep.

    ENSEMBL homology relationships are defined in:
    https://www.ensembl.org/info/genome/compara/homology_types.html

    Posible values to this function are '1:1', '1:n' and 'm:n'.

    >>> _get_relationships('1:1')
    ['ortholog_one2one']
    >>> _get_relationships('1:n')
    ['ortholog_one2one', 'ortholog_one2many']
    >>> _get_relationships('m:n')
    ['ortholog_one2one', 'ortholog_one2many', 'ortholog_many2many']
    """
    if notation == '1:1':
        return ['ortholog_one2one']
    if notation == '1:n':
        return ['ortholog_one2one', 'ortholog_one2many']
    if notation == 'm:n':
        return ['ortholog_one2one', 'ortholog_one2many', 'ortholog_many2many']
    raise ValueError('Orthology should be 1:1, 1:n or m:n')
