Crossovers, critical events in meiosis I, are the interchange of chromatid segments between paired homologous chromosomes (Fig. 1.3C). Segments of chromatids break and are exchanged as homologous chromosomes separate. As separation occurs, points of interchange are temporarily united and form an X-like structure, a chiasma (Fig. 1.3C). The approximately 30 to 40 crossovers (one or two per chromosome) with each meiotic I division are most frequent between genes that are far apart on a chromosome.

As a result of meiotic divisions, (a) genetic variability is enhanced through crossover, which redistributes genetic material, and through random distribution of homologous chromosomes to the daughter cells; and (b) each germ cell contains a haploid number of chromosomes, so that at fertilization the diploid number of 46 is restored.

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