Barr body

Barr body In , Barr and Bertram first observed a condensed body in the nerve cells of female cat which was absent in the male. This condensed body was called sex chromatin by them and was later referred as Barr body . In the XY chromosomal system of sex determination, males have only one X chromosome, whereas females have two. A question arises: how XII Std Zoology Chapter XII Std Zoology Chapter Principles of Inheritance and Variation does the organism compensate for this dosage differences between the sexes? In mammals the necessary dosage compensation is accomplished by the inactivation of one of the X chromosome in females so that both males and females have only one functional X chromosome per cell. Mary Lyon suggested that Barr bodies represented an inactive chromosome, which in females becomes tightly coiled into a heterochromatin, a condensed and visible form of chromatin (Lyon’s hypothesis). The number of Barr bodies observed in cell was one less than the number of X-Chromosome. XO females have no Barr body, whereas XXY males have one Barr body. • The number of Barr bodies follows N- rule (N minus one rule), where N is the total number of X chromosomes present.