Genetic control of the expression of allelic Ig genes at the VHa locus in a1/a2 heterozygous rabbits

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Abstract
Heterozygous rabbits representing 9 of 15 possible a1 and a2 heavy 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−n81f73g74de12,15 heavy 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 a1 is encoded by 4 other heavy chain haplotypes. For example, the mean a1: a2 ratio for adult a1x−y−n81f73g74de12,15/a2x32y33,− n82,f71g75de12,15 rabbits was 12:1 compared to 5:1 for a1x−y33,30n83,f71g75de12,15/a2x32y33,− n82,f71g75de12,15 heterozygous 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 VH genes and/or J gene segments, a separate regulator gene, or more efficient joining of certain gene segments, has yet to be determined.