During meiotic prophase in male mammals, the sex chromosomes are transcriptionally inactivated and form a condensed chromatin domain known as the sex body. It is not known how the assumption of this chromatin configuration is determined and regulated. We used various genetic models to test whether a complete sex-chromosome pair, effective sex-chromosome pairing, or an intact X chromosome is required for sex-body formation or transcription inactivation. The sex chromosome aberrations studied did not interfere with sex-body formation, and there is no evidence for inactivation failure or reactivation of the aberrant sex chromosomes. The results of this study suggest that control of sex-body formation is not intrinsic to the sex chromosomes and thus may be at the level of the testis.