Analysis of closely spaced COs.

Pairs of adjacent COs were analyzed in two wildtype tetrads genotyped by sequencing (wtx29 and wtx30) and in 46 wildtype tetrads genotyped by microarray (Mancera, 2008). A list of all possible COs was first generated using CrossOver, with the threshold for merging closely spaced events set to 0 kb. This list of COs was then analyzed to determine how many chromatids were involved in each pair of adjacent COs. The expected ratio for true COs in wild-cells is 1∶2∶1, which reflects a lack of chromatid interference. Note that the total number of COs in the category "COs at least 5 kb apart" is slightly larger than the number of COs found when CrossOver is run with a 5 kb threshold for merging closely spaced events, due to merging of some COs in the latter case; the exact positions of merged COs are also different. For each pool of events, a chi-square test was performed to determine whether the observed ratio deviates significantly from the expected ratio of 1∶2∶1.

本研究对2个经测序分型的野生型四分体（wtx29与wtx30）以及46个经微阵列（microarray）分型的野生型四分体（Mancera等，2008）中的相邻交叉重组（crossover, CO）对展开分析。首先使用CrossOver软件生成所有潜在交叉重组的列表，其中紧密相邻事件的合并阈值设为0千碱基对（kb）。随后对该交叉重组列表进行分析，以确定每对相邻交叉重组所涉及的染色单体数量。野生型细胞中真实交叉重组的预期比例为1∶2∶1，这一比例反映了无染色单体干涉（chromatid interference）的现象。需注意，"间隔至少5 kb的交叉重组"类别下的总交叉重组数量，略高于使用5 kb合并阈值运行CrossOver时检测到的交叉重组数量，这是因为后一种场景下部分交叉重组会被合并；且被合并的交叉重组的精确位置也存在差异。针对每个事件集，均执行卡方检验（chi-square test），以判断观测到的比例是否显著偏离1∶2∶1的预期比例。