<prompt> All are true about Lyons hypothesis except?Only one of X chromosomes is genetically activeOnly X chromosome from the mother undergoes pyknosisInactivation occurs on 16th day of embryonic lifeGene Xist causes gene silencing  <response>: Ans. b Only X chromosome from the mother undergoes pyknosisRef: Robbins pathology 9th ed. 164Lyon hypothesis is: Only one of the X chromosome is genetically active Other X chromosome which can be of paternal or maternal origin undergoes pyknosis and is rendered inactive. Inactivation of either paternal or maternal X chromosomes occurs at random among the cells of blastocyst by about 16th day of embryonic life Inactivation of X is because of a gene called Xist which is causing gene silencing DNA methylation. 