Y-chromosome in Drosophila composed of heterochromatin, but it contains 6 active loci for male fertility. Has a spreading effect that inactivates genes in a linear fashion in the adjacent euchromatin. Inactivating event occurs early in development, so that all daughter cells that are derived from an affected cell are deactivated. Discuss dosage compensation and X-chromosome inactivation in mammals. There are examples of non-randomness in certain tissues.
Inactivation occurs early in development due to the presence of mosaics.
Once inactivation occurs, the state is inherited somatically since clone cells all express the same allele.