Advanced Placement (AP) Biology Practice Exam

What does X inactivation refer to in females?

1. Randomly activates one X chromosome

2. Randomly inactivates and coils an X chromosome

3. Causes an abnormal number of chromosomes

4. Leads to the duplication of chromosomes

The correct answer is: Randomly inactivates and coils an X chromosome

X inactivation refers to the process in female mammals where one of the two X chromosomes is randomly chosen to be inactivated during early embryonic development. This inactivation results in the condensation of the chosen X chromosome into a structure known as a Barr body, effectively silencing its genes. This mechanism helps to equalize the dosage of X-linked genes between males (who have one X chromosome) and females (who have two). Thus, in females, the inactive X chromosome does not contribute to gene expression, allowing for dosage compensation. This random selection and the subsequent coiling into a Barr body is a crucial aspect of mammalian genetics that ensures balanced expression of X-linked genes. The other options do not accurately describe the phenomenon. The idea of randomly activating one X chromosome does not reflect the nature of X inactivation, as it specifically involves inactivation rather than activation. An abnormal number of chromosomes or the duplication of chromosomes relates to chromosomal abnormalities or errors in cell division, not specifically to the inactivation process of one X chromosome in females.