Estimating the fraction (

*k*) of three-strand DCOs that erase a crossover at

*GFP**. (A) Schematic representations of three-strand DCOs, all of which give rise to a tetratype (T) configuration of

*RFP* and

*CFP*. A subset has the same configuration as noncrossover

*GFP** conversion plus an incidental exchange (T

^{NCO}), and the remainder look like tetrads with just a single crossover (T

^{CO}). Which configuration arises depends on three factors: which

*gfp** allele is converted to

*GFP**, whether the incidental exchange occurs to the right or the left of the crossover at

*GFP**, and which chromatids are involved in the two crossovers. Each situation is diagrammed separately. Events diagrammed inside the brackets (Right) are three-strand DCOs where the incidental exchange involves the gene-converted chromatid’s sister. In the absence of chromatid interference, these events are expected to occur at twice the frequency of three-strand DCOs where the incidental exchange involves the gene-converted chromatid not bracketed. (B) The fraction (

) of three-strand DCOs expected to have a T

^{NCO} configuration is calculated for each converted allele separately. For conversion of

*gfp*-R215X*, a T

^{NCO} configuration is expected for one-third of the tetrads with the incidental exchange located to the right side and for two-thirds of the tetrads that have incidental exchange located to the left. Thus,

= [1/3⋅(right exchange) + 2/3⋅(left exchange)] ÷ total. Similar logic is applied to calculate the fraction of T

^{NCO} for conversion of

*gfp*-atg* (

). To estimate the likelihood of right-side incidental exchange

*vs.* left-side, we considered scenarios that ignore crossover interference or that assume strong interference (see Appendix text and B, Right). (C)

*k* is estimated by multiplying the fraction of three-strand DCOs that appear as T

^{NCO} for each allele (

) by the fraction of all conversions that are converted for that allele (

) and then summing the contributions for the two alleles.

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