Figure 1.

Outcomes of crossing over in Drosophila females heterozygous for a paracentric inversion. In each panel, only the innermost nucleus following meiosis II will become an oocyte nucleus and participate in fertilization. Without crossing over, homologs and sister chromatids segregate normally at the first and second meiotic divisions, respectively. Depending on initial orientation, either homolog has an equal probability of segregating to the oocyte. A single crossover within the inversion produces two noncrossover chromatids, an acentric fragment that is lost, and a dicentric bridge that is relegated to the two central nuclei at meiosis II; only a noncrossover chromatid can segregate to the oocyte. Thus, single-crossover progeny are not recovered from inversion heterozygotes and there is no increase in egg mortality. A double crossover involving the same two nonsister chromatids (two-strand double) results in two double recombinant and two noncrossover chromatids. The double recombinant chromatids can segregate normally to the oocyte nucleus. However, a double crossover involving all four strands (four-strand double) produces two acentric fragments and a double dicentric bridge. As shown, both chromatids composing the dicentric remain stuck in the central nuclei, resulting in an oocyte nucleus that is nullo-X and will produce a patroclinous male when fertilized by an X-bearing sperm. The known ratios of two-, three-, and four-strand double crossover bivalents allowed Sturtevant and Beadle to make their famous prediction that the ratio of double recombinant progeny to patroclinous males should be 3:2.