During meiosis, homologous chromosomes exchange segments via the formation of crossovers. This phenomenon is highly regulated, in particular, crossovers are distributed heterogeneously along the physical map and rarely arise in close proximity, a property referred to as interference. Crossover positions form patterns that give clues about how crossovers are formed. In several organisms including yeast, tomato, Arabidopsis, and mouse, it is believed that crossovers form via at least two pathways, one interfering, the other not.