Correct option is D
In females, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development, a process called X-chromosome inactivation. This ensures dosage compensation between males (XY) and females (XX). The inactivated X-chromosome forms a condensed structure known as the Barr body. As X-inactivation is random in each cell, females become mosaics with some cells inactivating the paternal X and others inactivating the maternal X.
Information Booster:
- X-inactivation occurs in early embryonic development, not immediately after fertilization.
- It is random, meaning either the maternal or paternal X is inactivated in each cell.
- The inactivated X forms a Barr body, visible in the nucleus.
- This process ensures equal expression of X-linked genes between males and females.
- Females exhibit a mosaic pattern of X-linked gene expression due to random inactivation.
Additional Knowledge:
- Option (a):
Incorrect. X-inactivation does not occur immediately after fertilization but takes place during the blastocyst stage of embryonic development. - Option (b):
Incorrect. X-inactivation is a random process in each cell, and there is no global preference for maternal or paternal X inactivation. - Option (c):
Incorrect. The Barr body is a condensed and inactivated X-chromosome, not the Y-chromosome. The Y-chromosome does not undergo inactivation. - Option (d):
Correct. Female mammals are mosaics for X-linked gene expression, with different cells expressing genes from either the maternal or paternal X-chromosome, depending on which X is inactivated.
