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.
Barr body, chromosome, dosage compensation, lyonization, mosaicism, sex linkage, tortoiseshell cat, X-linked trait