An unstable minichromosome comprising part of the short arm of chromosome 10 of maize was recovered from an oil yellow variegated plant as a consequence of gamma irradiation of pollen. The cytological and gene dosage observations are consistent with the minichromosome being a partial isochromosome, which lags at mitotic and meiotic anaphase. Loss of the minichromosome, which carries two doses of the +gene, causes phenotypic variegation in otherwise yellow lethal (Oy/Oy or Oy/oy) and olive (Oy/+ or oy/oy) genotypes. The minichromosome was transmitted to 8.1% of progeny via the pollen and 0.5% via the egg. Variations in the number and size of the minichromosome were recovered in progeny from a large test cross designed to test the feasibility for the detection of genetic variants including apomicts. No apomicts were recovered. All progeny with the appropriate maternal olive phenotype and the paternally derived coloured aleurone proved to be haploids. The recovery of a large minichromosome provides evidence for rare pairing and exchange with the short arm of chromosome 10. The variants of chromosome 10S generated from this programme provide useful material for further cytological, genetic and molecular analysis.