Random inactivation of X chromosome is:

Ans. A. LyonisationTwo factors that are peculiar to the sex chromosomes:(1) lyonization or inactivation of all but one X chromosome and (2) the modest amount of genetic material carried by the Y chromosome.In 1961, Lyon outlined the idea of X-inactivation, now commonly known as the Lyon hypothesis. It states that (1) only one of the X chromosomes is genetically active, (2) the other X of either maternal or paternal origin undergoes heteropyknosis and is rendered inactive, (3) inactivation of either the maternal or paternal X occurs at random among all the cells of the blastocyst on or about day 16 of embryonic life, and (4) inactivation of the same X chromosome persists in all the cells derived from each precursor cell.The inactive X can be seen in the interphase nucleus as a darkly staining small mass in contact with the nuclear membrane known as the Barr body, or X chromatin. The molecular basis of X inactivation involves a unique gene called XIST, whose product is a noncoding RNA that is retained in the nucleus, where it "coats" the X chromosome that it is transcribed from and initiates a gene-silencing process by chromatin modification and DNA methylation. The XIST allele is switched off in the active X.