The mushroom (champignon) is represented by two similar varieties that show differences in a process called crossing over during sexual reproduction. In commercial lines and most wild strains, the crossovers are mainly confined to the ends of the chromosome, while in the other variety crossovers occur throughout the chromosome. This has been one of the main obstacles in mushroom industries to improve or generate new mushroom lines because crossovers are needed for removing unwanted characteristics. We used different populations to study the mechanisms behind these two crossover patterns. We identified regions in the genome of mushroom revealing the difference in crossover patterns.