code: def iter_genotypes(self):
          
          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)
docs: Iterates on available markers.
  
          Returns:
              Genotypes instances.