The frequency of meiotic gene conversion often varies linearly from one end of the gene to the other. This phenomenon has been called 'polarity'. In this review, we will primarily studies of polarity that have been done in the yeast Saccharomyces cerevisiae (ARG4 and HIS4 loci) and in Ascobolus (b2 locus) with an emphasis on possible mechanisms. The genetic and physical data obtained at these 'hotspots' of recombination strongly suggests that the formation of a polarity gradient reflects both the frequency of heteroduplex formation and the processing of this recombination intermediate by mismatch-repair-dependent processes.