In eukaryotes facultative heterochromatization (FH) may serve for the genetic inactivation of whole chromosomes. The best known case is that of the X chromosome forming the Barr body in the mammalian female. The proposed study will analyze the two other known cases: the FH of the X during spermatogenesis, and that of a whole set of chromosomes of paternal origin in male mealybugs. The FH of the X during spermatogenesis is the most widespread and it has been suggested that it gave rise to the other two cases and thus that in all three the underlying molecular mechanism is similar. The object of the study is thus to determine the features common to all three cases. The FH of the X during spermatogenesis will be studied in the grasshopper Melanoplus femur-rubrum in which I recently determined that in tetraploid spermatocytes only one of the Xs present is heterochromatic. I also determined that these tetraploid spermatocytes are arrested during telaphase I. The possibility that the arrest of these cells was due to the genetic activity of the euchromatic X will be examined using tritiated uridine and autoradiography. The possible role of breakage in causing the recently described localized euchromatization of the X, and the interaction between the X and a heterochromatic B chromosome will also be examined. The FH of a whole set of chromosomes will be analyzed in the mealybug Pseudococcus obscurus by inducing the transmission of heterochromatic chromosomes from males into female embryos and determining whether the chromosomes remain heterochromatic or whether they become euchromatic. The role of the euchromatic chromosomes in maintaining the paternal set in the heterochromatic state will be analyzed by determining the behavior of the paternal set in the haploid sector of male embryos which are haplo-diploid mosaic, and in embryos which receive less than the full set of maternal chromosomes.