Return the actual genotype bases, e.g. if VCF genotype is 0/1,
        could return ('A', 'T')

def gt_bases(self):
        """Return the actual genotype bases, e.g. if VCF genotype is 0/1,
        could return ('A', 'T')
        """
        result = []
        for a in self.gt_alleles:
            if a is None:
                result.append(None)
            elif a == 0:
                result.append(self.site.REF)
            else:
                result.append(self.site.ALT[a - 1].value)
        return tuple(result)