Meiotic crossing-over: obligation and interference.

During meiosis, crossing-over--the exchange of genetic material between maternal and paternal chromosomes--is stringently controlled to restrict the number of crossovers per chromosome pair. In this issue of Cell, Martini et al., (2006) report that the reduction of crossover-initiating events does not result in fewer crossovers. These results have important implications for our understanding of crossover control.