Fetal sex can be diagnosed by noting the presence or absence of the Barr body in cells obtained from the amniotic fluid. What is the etiology of the Barr body?: (A) Inactivation of both X chromosomes, (B) Inactivation of homologous chromosomes, (C) Inactivation of one Y chromosome, (D) Inactivation of one X chromosome
Answer is D. The Barr body is formed from inactivation of one X chromosome in a female. The female has evolved a mechanism for permanent inactivation of one of the X chromosomes presumably because a double dose of X chromosome products would be lethal.