def iter_genotypes(self):
        """Iterates on available markers.

        Returns:
            Genotypes instances.

        """
        for v in self.get_vcf():
            alleles = {v.REF} | set(v.ALT)

            if self.quality_field:
                variant = ImputedVariant(v.ID, v.CHROM, v.POS, alleles,
                                         getattr(v, self.quality_field))
            else:
                variant = Variant(v.ID, v.CHROM, v.POS, alleles)

            for coded_allele, g in self._make_genotypes(v.ALT, v.genotypes):
                yield Genotypes(variant, g, v.REF, coded_allele,
                                multiallelic=len(v.ALT) > 1)