Final answer:
The correct statement about X-inactivation is that once a cell inactivates a specific X chromosome, all its progeny cells maintain the same inactivation pattern. This ensures dosage compensation between males and females.
Step-by-step explanation:
The true statement regarding X-inactivation is: b. After X-inactivation happens in a cell, all daughter cells resulting from mitotic events of that cell will have the same X chromosome inactive. X-inactivation is a process where one of the two X chromosomes present in the somatic cells of females becomes inactivated, ensuring that both males and females have a single functioning X chromosome in each cell. This inactivation is crucial in compensating for the double genetic dose of the X chromosome found in females. It starts early in development when the female mammalian embryo has only a few thousand cells.
The choice of which X chromosome becomes a Barr body is random within each cell but is then passed down to all descendant cells, maintaining the same X chromosome in an inactive state. This system ensures dosage compensation for genes on the X chromosome and is an example of an epigenetic mechanism that regulates gene expression, involving modifications like DNA methylation and histone hypo-acetylation leading to tight nucleosome packing.