Fungal tetrads and/or octads have been particularly advantageous because they have allowed geneticists to a) prove that crossing over occurs at the 4-strand (a.k.a. 4-chromatid) stage. b) genetically map the position of centromeres (via detection of "2nd division segregation" in linear asci). c) calculate more accurate map distances via the Perkins formula (which accounts for the effects of double crossovers that would go undetected with "random strand" analysis). d) all of the above