An effective approach is described for determining the B locus genotypes in a strain of outbred chickens. Matings were established within the strains studied to produce pedigreed offspring. A portion of the embryos produced was injected with blood from the sire or dam and their spleens were observed for graft-versus-host response. In other cases chicks were hatched and grafted with skin or wattle tissue from both their parents and two full sibs. Subsequently, grafted chicks were immunized with parental erythrocytes known to be incompatible for one B allele based on the results of the skin grafts and graft-versus-host studies. Eventually, antisera specific for B locus antigens were obtained. In Obese strain (OS) birds that spontaneously develop autoimmune thyroiditis two B alleles, B1 and B1, were identified with approximately equal gene frequency. A third allele, B3, was identified and observed only rarely (r = 0.04). Only one of the frequent alleles of OS chickens, B1, was identified in the parent White Leghorn Cornell C strain (CS) population from which OS chickens were selected. The frequency of B1 was similar in both strains. The B3 allele and another allele, B2, were present in moderate frequency in CS. Matings of B1B4 OS heterozygotes resulted in the segregation of offspring into the expected genotype ratio of 1 B1B1:2 B1B4:1 B4B4, verifying that the sera were specific for antigens determined by alleles of one locus. Several antisera specific for B locus antigens produced in other lines of chickens were used to test crythrocytes of OS and CS birds having different B genotypes. The agglutination results obtained with these sera substantiated the specificity of the antisera for B locus antigens.
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