This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
All are true about Lyon's hypothesis except? 
Select and explain. 
A. Only one of X chromosomes is genetically active, B. Only X chromosome from the mother undergoes pyknosis, C. Inactivation occurs on 16th day of embryonic life, D. Gene Xist causes gene silencing
B: Only X chromosome from the mother undergoes pyknosis
Answer: B: Only X chromosome from the mother undergoes pyknosis 
Explanation: Ans. (b) Only X chromosome from the mother undergoes pyknosisRef: Robbin's 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.