 Two isoforms are in the cis-haplotype pairing (AB and ab) and 2 are in the trans-haplotype pairing (Ab and aB).Such a person is a double heterozygote for these genes, for DQ the most popular situation.If a person carries haplotypes -A-B- and -A-b- then they can only make 2 DQ (AB and Ab), but if a person carries haplotypes -A-B- and -A-B- then they can only make DQ isoform AB, called a double homozygote.