Genetic Basis of Formation
The state of sex chromosomes in somatic cell nuclei depends directly on an individual's sex. This difference is driven by genetic compensation mechanisms.
- In males (XY karyotype): Both sex chromosomes—X and Y—remain in an active working state. In the nucleus, they appear as euchromatin, meaning their structure is decondensed (uncoiled), allowing genetic information to be transcribed. Sex chromatin is not normally formed.
- In females (XX karyotype): The situation is fundamentally different. For normal cell function, only one X chromosome must remain active—it is decondensed and represented by euchromatin. The second X chromosome, however, undergoes severe compaction. It transitions into an inactive state and becomes facultative heterochromatin.
This second, tightly condensed (inactivated) X chromosome in the somatic cell nuclei of females is called sex chromatin or the Barr body. It is not permanent (constitutive) heterochromatin, but rather facultative, as its formation stems from the necessity to "turn off" the extra chromosome.
Detection Methods and Study Objects
Because the Barr body is a compact and dense structure, it can be easily visualized using standard microscopic techniques. In laboratory practice, two main types of somatic cells are traditionally used to screen for sex chromatin.
- Neutrophilic leukocytes: A routine blood smear is prepared for this purpose. In neutrophils, sex chromatin takes on a very characteristic shape: it appears as a small appendage protruding from one of the segments of the multilobed nucleus. Due to its shape, this appendage is called a "drumstick" (drumstick).
- Oral mucosal epithelium: This is an excellent alternative object for investigation. Epithelial cells are easily obtained via a gentle scraping of the inner cheek. In the nuclei of these squamous cells, the Barr body is also clearly visible as a compact clump of chromatin, typically apposed to the nuclear envelope.
Diagnostic Significance
Determining sex chromatin is not only a fundamental biological phenomenon but also an important diagnostic tool. Its primary field of application is forensic medicine.
Analysis for the presence of Barr bodies (or leukocyte "drumsticks") allows investigators to determine the sex of biological material left at a scene with high accuracy. For example, experts can reliably distinguish bloodstains belonging to a female from those of a male, which often plays a pivotal role in criminal investigations.