2 MHC CLASS-I AND 2 MHC CLASS-II GENES MAP TO THE CHICKEN RFP-Y SYSTEM OUTSIDE THE B-COMPLEX

Gene sequences highly similar to major histocompatibility complex (Mhc ) class I and class II genes were recently recognized as mapping to a site in the genome of the chicken separate from the Mhc class I, class II, and B-G genes of the major histocompatibility (B) complex. The pr esent study was undertaken to see whether this complex of Mhc-like gen es designated as restriction fragment pattern Y (Rfp-Y) might reside i n one of three clusters of cosmid clones contained within the molecula r map of chicken Mhc genes, since only two of the three clusters can b e assigned to the B system. To determine whether the third cluster (cl uster II/IV) might contain Rfp-Y, a subclone (18.1) from within cluste r II/IV near a polymorphic lectin gene was used to analyze the DNA of families in which Rfp-Y haplotypes are known to be segregating. The re striction fragment polymorphisms revealed by the 18.1 probe were found to segregate in parallel with the restriction fragment polymorphisms defining the Rfp-Y haplotypes, thus establishing the location of Rfp-Y within cosmid cluster II/IV. Two of six Mhc class I genes and two Of five Mhc class II genes map to cosmid cluster II/IV, so a substantial fraction of chicken Mhc genes, including at least one that may be expr essed, are located in a chromosomal region separate from the B system. In further linkage analyses, Rfp-Y was found to assort independently from more than 400 markers in the present linkage map of the chicken g enome.