The oat (Avena sativa L.) cultivars, Steele and Dumont, differ by a chromosome interchange. Two tightly linked or allelic crown rust resistance genes (Pc-63 and Pc-38) are located on one of the segments involved in the interchange. Recombinant chromosomes can result from crossing-over in the region between the crown rust resistance locus and the interchange breakpoint. Our objectives were to use a diallelic duplicate locus to (i) identify recombinant [BC.sub.i][F.sub.1] genotypes with Pc-63 in the interchanged position, (ii) estimate the linkage relationship between the crown rust resistance locus and the interchange breakpoint, and (iii) recover duplication genotypes that possess four copies of Pc-63. Segregation of [BC.sub.1][F.sub.2] families was used to detect recombination within an interchanged chromosome segment in [F.sub.1] interchange heterozygotes. Only 3/7 of the single crossover recombinant gametes could be recognized by duplicate dominant segregation in [BC.sub.1][F.sub.2] families. After inoculation of 145 [BC.sub.1][F.sub.2] families with a crown rust (Puccinia coronata Corda var. avenae W.P. Fraser & Ledingham) isolate avirulent on Pc-63 and virulent on Pc-38, we identified 11 recombinant families segregating 15R:1S. A 15:1 segregation ratio indicated Pc-63 recombined to an interchanged position as a result of crossing-over between the crown rust resistance locus and the interchange breakpoint. Our data indicated Pc-63 was linked in repulsion and distal to the interchange breakpoint with 17.7% recombination observed between the crown rust resistance locus and the breakpoint. Testcross segregation ratios confirmed that recombination occurred and allowed identification of a true breeding duplication genotype that possessed four copies of Pc-63. Increasing the dosage of Pc-63 did not alter the expected infection type conferred by a one or two copies of Pc-63., Many hexaploid oat (Avena sativa and A. sterilis L.) genotypes differ by chromosome interchange differences (McMullen et al., 1982; Leach, 1989; Singh and Kolb, 1991). Although cytologically identifiable, the genetic [...]