Final answer:
About 15-25% of genes on the inactivated X chromosome escape inactivation, ensuring essential gene expression and contributing to genetic diversity. This selective inactivation is vital for maintaining gene-dosage balance and is exemplified in tortoiseshell cats' coat color patterns.
Step-by-step explanation:
Not all of the X chromosome is subject to inactivation; there are specific regions that escape inactivation. It's estimated that about 15-25% of the genes on the inactivated X chromosome (Xi) escape inactivation and continue to be expressed. This phenomenon is crucial as it allows for the X chromosomes to partly circumvent the silencing process and contribute to the genetic diversity that accounts for variable phenotypes. The escape from inactivation occurs because some regions on the X chromosome are rich in genes that are indispensable for survival. Furthermore, X chromosomes must reactivate for proper maturation of female ovaries, indicating that complete inactivation across the entire chromosome is not viable. This selective inactivation ensures that gene-dosage balance is maintained across the cells.
For example, in tortoiseshell cats, the mosaic pattern of coat colors directly illustrates how certain genes on the X chromosome can escape inactivation. The cells where one X chromosome encoding for orange fur is inactivated will express the other, potentially black fur gene, and vice versa. Thus, females express one of two different coat colors over different regions of their body, demonstrating the selective inactivation of different X chromosomes in different regions.