The most straightforward method to recover chromosomes carrying an extra copy of a segment is transposition, where three chromosome breakpoints allow the movement of a segment to a new insertion site as shown here.
It is important to note the distinction between a transposition and a duplication. Though often used interchangeably, a transposition (Tp) consists of a deletion (Df) and a duplication (Dp). The duplication component of an intrachromosomal transposition can be separated from the deletion component by meiotic recombination. Intrachromosomal duplications are denoted Dp(1;1), Dp(2;2), etc.
|2. Tandem duplication|
Many duplications arise from nearby breaks on homologous chromosomes (as shown below) or on sister chromatids.
Tandem duplications tend to be unstable and revert to single copies by exchange between repeated copies. For this reason, tandem duplications are usually maintained in stock in combination with a mutation or deletion of a haplolethal or haplosterile gene in the same region.
|3. Recombination between overlapping inversions|
Intrachromosomal duplications can be generated from overlapping inversions as shown here.
When meiotic recombination occurs within the region shared by overlapping inversions, the resulting chromosomes will be duplicated or deleted for the regions between the two left breakpoints and the two right breakpoints. A chromosome can have two duplications, two deletions or a deletion and a duplication depending on the relative positions of the breakpoints.
Because deletion and duplication of centric heterochromatin is usually inconsequential, inversions with proximal breakpoints located in centric heterochromatin are often used to simplify this approach.
The symbols for the resulting chromosomes reflect the left side of one inversion and the right side of the other as shown above. Sometimes Df or Dp synonyms are substituted for these complicated symbols. For example, Dp(2;2)Cam2 was originally called In(2LR)DTD24[L]DTD8[R].
And for you chromosome geeks...
Many intrachromosomal duplications of this type were made from pairs of pericentric inversions using autosynaptic intermediates. The method is conceptually equivalent to the figure above, so you don't really need to know the details, but, if you're curious, it is really, really cool genetics! See chapter 20 of Drosophila: a laboratory handbook and Gubb (1998).
|4. Combining translocation segregants|
Each “half” of a reciprocal translocation is called a translocation segregant and receives a symbol showing the chromosome tips it carries. Translocation segregants from different reciprocal translocations can be combined to duplicate or delete chromosomal segments as shown here.
Translocation segregants with centric heterochromatin breakpoints are often used to simplify this approach so that only one contiguous block of genes is deleted or duplicated. The deletion or duplication of centric heterochromatin usually presents no problems.
Because the symbols for translocation segregant pairs are long and complicated, synonyms are often used. For example, Dp(3;3)Cam30T is a synonym for Ts(2Lt;3Lt)el24 + Ts(2Rt;3Rt)TE35B-28.