If a diploid organism wishes to reproduce sexually, one of the problems that is encountered is how to reduce the chromosome number such that the gametes have one of each chromosome, so that when two gametes fuse they produce a diploid zygote. This is achieved through a special pair of cell divisions called meiosis.
Mistakes result in aneuploidy, which is frequently lethal.
There are many ways in which accurate chromosome segregation is achieved. The most common is by crossing over. First, homologous chromosomes pair and recombine with one another. Some of the recombination events become crossovers. The sites where crossovers have occured become chiasmata, cytologically visible connections between chromosomes.