Two of the four strands of a bivalent are recovered together in tetraploid progeny arising from 4x-2x matings. This provides a method for gene and centromere mapping. The cross pppp x Pp produced 62 nulliplex tetraploid individuals in a total of 951, i.e., 6.5%. The diploid clone was found to produce diplandrous gametes through first division restitution (FDR). The map distance P-centromere was estimated as 13.0 units, the limiting values at the 95% binomial confidence interval being 10.1 and 16.3. The mapping method is explained in detail and a formula is deduced to obtain genotypic series of 2n gametes under particular assumptions. The direction 4x×2x is advantageous, since high seed set diploid clones which give 99% tetraploid progeny, and whose method of diplandroid formation is known are already available. Diploid clones heterozygous for many markers and tetraploids nulliplex for the same markers are needed to fully exploit this method.