Chromosomal crossover is the process by which two chromosomes, paired up during Prophase I of meiosis, exchange some distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but to the different end piece. If they break at the same place or locus in the sequence of base pairs, the result is an exchange of genes. This outcome is the normal way for crossover to occur. If they break at slightly different loci, the result can be a duplication of genes on one chromosome and a deletion of these on the other. If they break and rejoin on opposite sides of the centromere, the result can be one chromosome being lost during cell division. If homologous chromosome are not perfectly aligned an unequal crossover may occur, an unequal crossover can create duplicate genes.
Any pair of homologous chromosomes may be expected to cross over three or four times during meiosis. This aids evolution by increasing independent assortment, and reducing the genetic linkage between genes on the same chromosome.