Barr bodies are typically found in the nucleus of which cells?

• A. In the cytoplasm of all cells
• B. In the nucleus of female somatic cells only
• C. In the mitochondria of male germ cells
• D. In the cytoplasm of female germ cells

Answer: (B)

The concept tested is X-chromosome inactivation, where one of the two X chromosomes in female cells becomes transcriptionally inactive and condenses into a dense Barr body located in the nucleus. In females, having two X chromosomes means dosage compensation is needed to balance gene expression with males who have one X. The inactivated X condenses into a Barr body and sits inside the nucleus of most female somatic cells. Males have only one X, so there isn’t a second X to inactivate, and Barr bodies aren’t formed. This structure is not in the cytoplasm or mitochondria, and during gametogenesis the inactivated X is typically reactivated in germ cells, so Barr bodies are not a characteristic feature of mature female germ cells.