The Lr62/Yr42 translocation comprises mostly allen chromatin and retains only the very distal end of wheat chromosome arm 6AL, including the telomere. The large amount of foreign chromatin prohibits commercial use of the resistance and an attempt was therefore made to exchange some of the introgressed chromatin for wheat chromatin. Plants heterozygous for the translocation, but lacking the Ph1 locus were testcrossed with Chinese Spring nullisomic 6A tetrasomic 6B or nullisomic 6A tetrasomic 6D plants. Resistant (Lr62) testcross F1 progeny were evaluated for the presence of three marker loci and the data used to do a three-point genetic mapping analysis. Forty one recombinants were identified and characterised with further markers. The data revealed that Lr62/Yr42 occurs towards the distal end of 6AS and that the 6AS telomeres of the wheat and translocation chromosomes were homoeologous. While the wheat (Chinese Spring ph1b mutant) and translocated chromosomes 6A shared sufficient homoeology to allow for regular allosyndetic recombination, there were also major structural differences between them, including a duplication (marked by Xgwm334) and translocation (marked by XsopwT). The duplicated region occurred on chromosome 6A of the CS ph1b mutant and probably resulted from its inherent genomic instability. The structural differences caused irregular meiotic pairing and complex segregation data that were difficult to interpret. It was, however, possible to explain the majority of recombination products in keeping with the expected low frequency of allosyndetic recombination, and to identify the most promising recombinants. These retained both Lr62 and Yr42 within a comparatively small region of foreign DNA at the 6AS telomere.Articl
