The relationship between chromatid interference and the mapping function.

A formula for a general mapping function involving only the number of chiasmata between two points on the chromosome and the average excess of four-stranded double exchanges over two-stranded double exchanges is derived. A formula is also given for deriving the proportion of three-stranded double exchanges after any given number of chiasmata; this is related to mapping for linkage with the centromere. It is shown that this proportion tends to a limit of 2/3 after a large number of chiasmata, whether or not chromatid interference is operating.