AbstractHeterozygous rabbits representing 9 of 15 possible a1and a2heavy chain haplotype gene combinations among rabbits in the University of Illinois colony were analyzed for ratios of al to a2 in serum immunoglobulin (Ig). The Ig from rabbits of the a1x−y−n81f73g74de12,15heavy chain haplotype in combination with any of three a2‐associated heavy chain haplotypes have higher ratios of a1 to a2 than Ig from rabbits in which a1is encoded by 4 other heavy chain haplotypes. For example, the mean a1: a2 ratio for adult a1x−y−n81f73g74de12,15/a2x32y33,−n82,f71g75de12,15rabbits was 12:1 compared to 5:1 for a1x−y33,30n83,f71g75de12,15/a2x32y33,−n82,f71g75de12,15heterozygous rabbits. Family studies indicated that the a1:a2 ratio was under the control of the heavy chain chromosomal region or a locus closely linked to it. Whether the regulation is due to varying numbers of VHgenes and/or J gene segments, a separate regulator gene, or more efficient joining of certain gene segments, has yet t
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