What is a Barr body?

A Barr body is specifically defined as an inactive X chromosome in female mammals. In organisms with two X chromosomes, such as female mammals, one of the X chromosomes undergoes a process called X-inactivation. This process is essential for dosage compensation, ensuring that females do not produce double the amount of X-linked gene products compared to males, who have only one X chromosome.

The inactivated X chromosome condenses and can be visualized as a dense mass within the nucleus, which is referred to as a Barr body. This phenomenon plays a crucial role in genetic regulation and development. The presence of a Barr body indicates that X-inactivation has occurred, generally leading to the inactivation of gene expression from that chromosome.

The other options pertain to different biological phenomena: immune cells are unrelated to this structure, mutated DNA refers to changes in the genetic sequence that can lead to diseases, and reproductive cells in males do not correspond to the concept of a Barr body. Each of these choices reflects distinctly different facets of biology that do not contribute to understanding X chromosome inactivation.