SUMMARYSerum IgG (7S) levels differed significantly for chickens from 10 different inbred lines. Within lines differences between B blood groups were statistically significant.The genetic control of serum IgG was further examined using birds fromBcomplex haplotypes marked at theBlocus and theIr‐GATlocus. Birds from each of five subgroup haplotypes (B1B1Ir‐GAT‐Loand‐Hi, B19B19Ir‐GAT‐Loand‐Hi, andB2B2Ir‐GATintermediate) were tested for levels of serum IgG at 3, 6, 9, and 21 weeks of age. The rate and level of IgG reached in the serum was more than two‐fold greater in the GAT‐Hi birds than in the GAT‐Lo. The Ir region of the B complex exerts some control over the ontogenesis of IgG, though most of the genetic variation seems not to be
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