Question

Describe how X-inactivation works during the blastocyst stage

Answer 1

In the early blastocyst, X-inactivation is reversed in the inner cell mass and in these cells both X chromosomes are active. Each of these cells independently and randomly inactivates one copy of the X chromosome. This inactivation event is irreversible during the lifetime of the cell. X-inactivation is an epigenetic change that results in a different phenotype, is not a change at the genotypic level. For a lineage, the inactivation is skewed, and give rise to mild symptoms in female carriers of X-linked genetic disorders. X-inactivation is reversed in the female germ-line, so that all oocytes contain an active X chromosome.