By the cytogenetic method of trisomy mapping, the major histocompatibility complex (MHC) (B) was located on the microchromosome that also contains all of the ribosomal ribonucleic acid (rDNA) genes that are detected as a nucleolar organizer region. Crosses involving aneuploid chickens homozygous or heterozygous for particular B haplotypes yield an F1 of disomic, trisomic, and tetrasomic offspring suitable for studies of gene dosage effects and gene regulation. Studies to date show that MHC genes are expressed on each chromosome in aneuploid cells unlike the rDNA gene cluster, which is regulated to produce only diploid levels of mature rRNA. Biological effects of extra MHC chromosome dosage range from altered cell surface content of glycoproteins to altered growth potential of chickens. In addition, enhanced MHC-encoded cell surface products may influence the progression of B-cell differentiation and cell population dynamics in the developing bursa of Fabricius. Recent research shows a possible mechanism to account for the formation of unequal products in meiosis within the rDNA and MHC gene clusters. Unequal recombinational events may be promoted in the meiotic process in trisomic chickens.