Evidence of preferential pairing of chromosomes at meiosis in aneuploid yeast.

Meiotic pairing in homothallic S. cerevisiae was studied by tetrad analysis, using strains that were trisomic or tetrasomic for chromosomes. I. The disomic segregants of these strains produce tetrasomic spore colonies that can be distinguished by their phenotype. Results indicated the existence of preferential pairing and nonrandom assortment of chromosomes at meisosis I. The frequency of crossing over is apparently normal in at least some regions when nonpreferred pairing occurs.