Unequal crossing over

[1] During meiosis, the duplicated chromosomes (chromatids) in eukaryotic organisms are attached to each other in the centromere region and are thus paired.

When two sequences are misaligned, unequal crossing over may create a tandem repeat on one chromosome and a deletion on the other.

[2] Unequal crossing over is the process most responsible for creating regional gene duplications in the genome.

With the increase in the duplicates, unequal crossing over can lead to dosage imbalance in the genome and can be highly deleterious.

Compared with gene conversion, which can only transfer a maximum of 1,500 base pairs, unequal crossing over in yeast rDNA genes has been found to transfer about 20,000 base pairs in a single crossover event[1][3] Unequal crossover can be followed by the concerted evolution of duplicated sequences.

[1] These short sequences have no selection pressure acting on them and the frequency of the repeats can be changed by unequal crossing over.