Easy summary of barr body

Barr body 

The condensed, inactive, single X-chromosome , appearing as a densely staining mass, that is found in the nuclei of somatic cells of female mammals. It is named after its discoverer, Murray Barr.

 The number of Barr bodies is thus one less than the number of X-chromosomes. 

The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis 

A genotypical human female has only one Barr body per somatic cell, while a genotypical human male has none.

This happens early in embryonic development at random in mammals  except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the father's X chromosome is always deactivated.

The human abnormalities called Kleinefelter's syndrome and Turner's syndrome both result from an unnatural presence or absence of a Barr body. 

Kleinefelter's syndrome(47XXY genetically male but contain Barr body  and

 Turner's syndrome (45X0 genetically female but contain no Barr body .