What is crossing over? How is meiosis related to this phenomenon?

Linked alleles, for instance, A-b and a-B, form the gametes A-b and a-B that maintain the linkage of the alleles. This type of linkage is known as complete linkage. In the first division of meiosis (meiosis I) however the crossing over phenomenon may happen. Chromosomes from a pair of homologous may exchange extremities and some once linked alleles, for instance, A-b and a-B, recombine to form dissimilar gametes, in the case, A-B and a-b.

Crossing over may occur when the arms of the chromatids of each homologous are paired during meiosis. Matching portions of the extremities of two nonsister chromatids (one from one homologous of the pair) break and the pieces are exchanged, each of them becoming part of the arm of the other chromatid. For instance, if the allele A is situated in a side of the arm relating to the point of breaking and the allele b is located in the other side, they will be separated and gametes A-B and a-b will be produced, instead of A-b and a-B.