A cross-over chromosome combiningIg heavy chain genes of two mouse strains

Abstract

Two congenic strains of mice were bred in Konstanz that bear theIg heavy chain allotype gene of the C57BL/6 (Ig-1 b) in a BALB/c background genome. One line (CB-8K) underwent eight backcross generations to BALB/c before sister-brother mating was initiated. For the other line (CB-16KN) backcrossing was continued for eight further generations, then a homozygousIg-1 b/Ig-1 b strain was produced by sister-brother mating. Both lines were tested for four VH markers of the BALB/c and one of the C57BL parent. The CB-16KN strain expressed the C57BL marker VHNPb together with the C57BL allotype marker, and failed to express the three BALB/c markers, VHDEX, VHS117, VHphOx and VHNPa. It thus resembled the CB-20 strain.Strain CB-8K expressed the VHNPb marker of C57BL but also all the four BALB/c markers that were tested. The strain appeared more heterogeneous than the CB-16KN strain, and a subline was bred from two exceptional mice that did not express the VHNPb marker. This subline (CB-8KN) expressed the BALB/c marker VHNPa regularly, and was negative for the VHNPb marker. It thus resembles the BAB-14 line.The crossing over event thus must have happened in one of the two meioses, which led to the CB-8K line. As BAB-14 is derived in an analogous manner to a branch of the backcross of Potter and Lieberman, which ended up in CB-20, the unexpected finding is discussed that two independent crossing over events (in CB-8KN and BAB 14) within theIg heavy chain gene region have taken place at approximately the same stage of two breeding programs.