The term “bar cell” is a common misunderstanding of the correct term, Barr body. A Barr body is not a cell, but a structure within the cells of female mammals. It is an X chromosome that has been rendered inactive and condensed, first identified in 1949 by Canadian scientist Murray Barr.
The Reason for X-Chromosome Inactivation
Inactivating an X chromosome is necessary due to the genetic difference between sexes. Females have two X chromosomes (XX), while males have one X and one Y (XY). The X chromosome carries many genes for bodily functions, and if both were active in a female, her cells would produce twice the proteins from these genes compared to a male’s cells.
This potential doubling of gene products could cause developmental issues. To prevent this imbalance, a process called dosage compensation “switches off” one of the two X chromosomes in every somatic (non-reproductive) cell of a female. This balances the genetic expression between the sexes.
Formation and Appearance of a Barr Body
The formation of a Barr body is a process called Lyonization, named after geneticist Mary Lyon. This occurs early in female embryonic development. In each cell, the choice of which X chromosome to inactivate is random, and that choice is permanent for all future cells that descend from it.
The inactivated X chromosome becomes highly condensed into a structure called heterochromatin. This compaction makes its genes inaccessible for transcription, silencing them. Under a microscope, the Barr body appears as a small, dense spot at the periphery of the cell’s nucleus.
Consequences of Random Inactivation
The random nature of X-inactivation explains the coat pattern of calico and tortoiseshell cats. The gene for fur color is on the X chromosome, with alleles for black and orange fur. Since these cats are female, if the X chromosome with the orange allele is inactivated in a patch of cells, that patch will be black. If the X with the black allele is turned off, the patch will be orange.
This principle is also used in genetic testing to identify conditions related to the number of sex chromosomes. Females with Turner syndrome (XO) have no Barr bodies, while males with Klinefelter syndrome (XXY) have one. The rule is that the number of Barr bodies is one less than the total number of X chromosomes in the cells.