Differential Gene Expression Differential gene expression is the means by which differentiation comes about. The zygote is totipotent and is capable of forming a complete adult. Depending on the species, the cells resulting from cleavage may be totipotent at earlier or later stages in development of the early embryo. An early theory concerning differentiation suggested that the cell nucleus undergoes permanent change during development. Hence, differentiation should be irreversible.
Irreversible differentiation is seen in mammalian erythrocytes and the tracheids of vascular plants.
In each case, the nucleus is missing in the functional cells and its absence can account for the irreversibility. Work by several researchers, Briggs and King , Gurdon , and Wilmut demonstrated that differentiation does not involve permanent changes in the genome. No information was lost as the nucleus passed through the early stages of development. Wilmut and his colleagues in Scotland were responsible for the research that produced Dolly, the famous cloned sheep. Their work was based on that of the previous researchers, but instead of frogs, the sheep were used.