Crossing over in meiosis results in genetic recombination, which is responsible for the genetic diversity of a population. Genetic recombination and natural selection are the driving forces behind evolution. It causes most of the differences between parents and their offspring and differences between siblings.
Meiosis, otherwise known as sex cell division, creates unique gametes, which are eggs in females or sperm in males, from existing DNA in eukaryotes. For meiosis to occur, the chromosomes contributed by each of the organism's parents are duplicated to form sister chromatids. During meiosis I, the sister chromatids of one parent match up with the corresponding sister chromatids of the other parent, or its homologous non-sister chromosome, along the metaphase plate. Crossing over then occurs. At a point called a chiasma, homologous chromosomes trade genetic information so that each chromosome is complete but has different information. This random exchange of information is what allows for unique gametes to form and genetic recombination to occur.
Sometimes, however, too few crossing over events can result in abnormally short or long chromosomes, meaning that there is too little or too much information. If a gamete with this type of chromosome is fertilized, abnormalities in the offspring are likely.