Possible Loss of Length Conservation and Reciprocity during Recombination or Conversion in Tandem Arrays

Recombination or conversion between arrays of repeated sequences need not be conservative for length because two single strands of DNA from one chromatid may form heteroduplexes in different registers on the other participating duplex. This can cause an overall change in repeat number. Loss of length conservation is equally possible whether models of recombination initiated by single-strand transfer or double-strand break are applicable. Length changing conversion will frequently produce a characteristic insertion within a deletion, often appearing as a double deletion such as are frequently found in new variants of human minisatellite MS32. There is no apparent means of preserving parity during length-changing conversion or recombination, and if the changes are biased then the overall copy number will increase or decrease according to that bias. The observation that arrays persist suggests that any bias in these arrays will be toward gains. An equilibrium may be reached where gains, which may be largely independent of array length, equal losses from the array-length-dependent processes of intrachromatid recombination and repeated unequal sister chromatid exchanges.