Crossing over

Crossing over occurs once chromosomal replication has occurred in meiosis, (i.e. when all the bivalents have 4 chromatids.) It is the process whereby homologous chromosomes exchange sections of genetic material. Crossing over is essential to the survival of a population as it contributes to genetic variation. 

Multiple crossing over

Multiple crossovers usually occurs when two genes are far apart from each other along a chromosome. It can happen in a single meiosis and the chance of getting multiple crossovers can be enhanced by increasing the distance between genes. Multiple crossing over is the reason for the recombination data within the chromosomes. There are several types of multiple crossing over, for example two-strand double crossing over, three-strand double crossing over and four-strand crossing over. It usually happens from independent assortment of the genes.