Unequal crossing over
Unequal crossing‑over occurs when pairing occurs of two mutually complementary but simultaneously nonhomologous DNA segments, with subsequent crossing-over between them. If unequal crossing-over occurs in thea same DNA molecule, this leads to the formation of a single circular DNA molecule and deletion of the relevant DNA section from the original molecule (Fig. VI.5).
Unequal crossing-overbetween two DNA molecules, e.g. between DNA segments lying on two different chromosomes, leads to duplication of the given segment on one chromosome and its deletion on the other chromosome (Fig. VI.6).
There is only apparent symmetry between duplication and deletion. Deletion is more frequently lethal than duplication, so that its bearers are more frequently removed from the population by natural selection. However, what is more important is that further duplication occurs with much greater probability in multiplied, e.g. duplicated, segments. This thus releases a positive feedback spiral that leads to the accumulation of an increasing number of duplicated segments in the genome of the organism and subsequently in the gene pool of the species.