Final answer:
Before X inactivation, both X chromosomes in female cells produce high levels of gene products to ensure similar expression levels of X-linked genes in males and females. X inactivation leads to the formation of a Barr body, maintaining dosage compensation by silencing one of the two X chromosomes in females.
Step-by-step explanation:
Before X inactivation (XIC) activation, both X chromosomes in female mammalian embryos produce high levels of gene products. This ensures that males and females express similar amounts of X-linked genes. However, during embryonic development, females undergo a process where one of the two X chromosomes in each cell becomes inactive or condenses into a Barr body. This epigenetic mechanism is crucial for dosage compensation, which ensures that females, who have two X chromosomes, do not produce twice as much of the proteins encoded by the X chromosome genes as males, who only have one.
Significantly, even the inactivated X chromosomes (Barr bodies) continue to express a few genes that are necessary for certain physiological processes, such as the proper maturation of female ovaries. Conditions that involve abnormalities in the number of X chromosomes are typically associated with mild mental and physical defects and sometimes sterility.