Deletions and duplications can be formed by unequal crossing-over between DNA sequences that are in direct repeats. (Duplications of adjacent sequences are called tandem duplications.) When this happens via intramolecular recombination (i.e. between sequences on the same DNA molecule), it probably occurs between sister chromosomes soon after the DNA has replicated. The following figures show the two consequences of unequal crossovers, drawn as half-crossover events to emphasize how duplications arise vs how deletions arise.
Because transposons provide portable regions of homology that can be inserted at specific regions of the chromosome, two copies of a transposon inserted in the same orientation on the chromosome can provide the direct repeats needed to duplicate or delete any desired region of the chromosome (except of course you cannot delete essential genes). Examples are shown below. The orientation of the transposon insertion is indicated by the direction of the arrowheads.
These duplications will be unstable because recombination between the direct repeats can delete the duplicated region (a process called segregation).
Last modified October 15, 2003