During the production of eggs and sperm, homologous chromosomes pair up and cross over, exchanging genetic material. Crossing over results in offspring that are genetically distinct from their parents, thereby shaping natural selection. This process is initiated by the formation of hundreds of programmed DNA double-strand breaks. However, only a few of them result in crossovers, with the majority of breaks being repaired without a crossover. The number and placement of crossovers has profound consequences for health and fertility, as errors in it lead to aneuploidy, a leading cause of pregnancy loss and intellectual disability in humans. To understand the rules governing this process, Hinch et al developed a novel method to sequence individual sperm and built a high-resolution map of crossovers in mice. They identified key factors that affect the time it takes for a break to find and engage with its homologous chromosome. Each of these factors also affects the probability of a break being resolved a crossover, such that breaks that are faster to find their homologues are more likely to be repaired as crossovers
The full paper can be read at Science, or in the print edition.