C. elegans has unique features for genetic studies as follows. (1) Fast reproductive cycle. Only 3 days is enough time to reach adult size from embryos. (2) Germ cells align spacio-temporal manner in the gonads. It is easy to access a lot of germ cells in the single gonad. Time course experiments are not required to synchronize the meiotic cell cycle. (3) The number of chromosome is low. C. elegans has only 12 chromosomes in a cell (n=6), so 6 pairs of bivalents are easily distinguished at diakinesis stage by fluorescent microscopy. (4) XX hermaphrodite and XO male. It is easy to maintain C. elegans strains. Because its standard sex is hermaphrodite, one worm can produce next generation individuals by self fertilization. It is important to note that XX hermaphrodites produce 99.8% XX hermaphrodite and 0.2 % XO male due to X chromosome nondisjunction. We can isolate mutants with meiotic defects by focusing on the ratio of male (him; high incidence of males) with embryonic lethality. (5) Tight regulation of crossover number and distributions. The number of crossovers is limited; there is only one crossover between each pair of homologous chromosomes. The single crossover is biasedly formed at the arm region of chromosomes and suppressed at the center region.