def METHOD_NAME(self):
    """Return the genotype index for unphased, diploid calls.
    Returns
    -------
    :obj:`int`
    """
    if self.ploidy != 2 or self.phased:
        raise FatalError(
            "'unphased_diploid_gt_index' is only valid for unphased, diploid calls. Found {}.".format(repr(self)))
    a0 = self._alleles[0]
    a1 = self._alleles[1]
    assert a0 <= a1
    return a1 * (a1 + 1) / 2 + a0