This animation shows how X chromosomes are randomly switched off in most female mammals.

In most female mammals, one X chromosome is randomly inactivated to compensate for the fact that females have twice as many X-chromosome genes as males do. This phenomenon is called dosage compensation. About half the cells in these females express genes from the maternal X chromosome, and the other half express genes from the paternal X chromosome. The animation illustrates this phenomenon, called X inactivation, with an example from calico cats.

This animation is a clip from a 2003 Holiday Lecture Series, Learning From Patients: The Science of Medicine. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different steps in the inactivation process.