Structural and genetic studies on chicken 7S immunoglobulin allotypes. II. Distribution of allotypes on the 7S immunoglobulin of homozygous and heterozygous chickens

We have previously reported that chicken 7S immunoglobulin (Ig) heavy (H) chain allotypes (CS 1 locus) segregate as phenogroups in F₂ progeny. Specificity CS 1.1 formed a phenogroup with CS 1.4 in inbred chicken line UCD 2, and a second phenogroup with CS 1.3 in line UCD 3. To determine whether these phenogroups were formed by combinations of specificities on the same, or on separate subclasses of 7S Ig, their distribution on the 7S Ig molecules of birds homozygous for 7S Ig allotypes was analyzed by radioimmunoassay. Anti CS 1.1 and anti CS 1.3 alloantisera each bound more than 94% of line UCD 3 ¹²⁵I 7S Ig. Similar results were obtained with alloantisera to CS 1.1 and CS 1.4 with ¹²⁵I 7S Ig from line UCD 2. These results indicate that both phenogroups were formed by combinations of specificities present on the same H chain. Thus, each phenogroup described, probably, is the product of a single structural gene which is responsible for more than 94% of the 7S Ig H chain constant regions. In F₁ hybrids with the genotype CS 1.1, 1.3/CS 1.2, two populations of serum 7S Ig molecules were detected by direct and sequential binding analysis with specific alloantisera. One population of 7S Ig contained specificities CS 1.1 and CS 1.3, but not CS 1.2; while the second population carried only specificity CS 1.2. Therefore, each population was exclusively the product of one parental allele. Consistent with a genetic regulatory mechanism involving allelic exclusion, no 7S Ig containing allotypes produced by both alleles was detected.