Crossing over is a process that occurs during meiosis. Prior to meiosis, a chromosome replicates. This results in two chromatids that are joined in the middle by a centromere. As the cell prepares for division, two homologous chromosomes align. Crossing over occurs when segments of the two aligned homologous chromosomes are exchanged randomly. Alleles from one chromosome will switch places with the corresponding alleles from the other chromosome at some point along the DNA molecule. This "recombination" is one way in which genetic variation is increased amongst individuals within a species.
The following article speaks towards the length between two genes and crossing over frequency. The longer the distance between two separate alleles, the higher chance that they will be separated by recombination. This is simply because of the fact that there is more space between them at which the crossover can occur, and these crossovers have no locational preference - once again, they simply occur randomly.