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

The 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 kind of linkage is called as complete linkage. In the first division of meiosis (meiosis I) though the crossing over phenomenon may occur. The Chromosomes from a pair of homologous may exchange extremities and some once linked alleles, for instance, A-b and a-B, recombine to form different gametes, in the case, A-B and a-b.

The Crossing over may happen when the arms of the chromatids of each homologous are paired during meiosis. The Matching portions of the extremities of two nonsister chromatids (one from one homologous of the pair) break and the pieces are exchanged, all 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 formed, instead of A-b and a-B.

(The percentage of the recombinant gametes relating to normal gametes depends upon the crossing over rate that in its turn depends upon how far distant the given alleles are in the chromosome.)