The sheep Dolly was the first cloned mammal. Dolly was created by implanting a nucleus from a cell taken from the udder of a female sheep into an enucleated egg. This nucleus had two X chromosomes, and because it came from a differentiated cell, one of them must have been inactivated. If the udder cell was heterozygous for at least one X-linked gene whose expression you could assay, how could you determine if all of Dolly’s cells had the same X chromosome inactivated?

If, upon testing, Dolly’s cells prove to be mosaic for X chromosome activity—that is, different X’s are active in different clones of cells—what must have happened during her embryological development?