|MadSci Network: Genetics|
This sort of question has intrigued many science fiction fans, as well as scientists, since the function of genes was discovered. The X-Men movie series generated even more interest, as in Mark Silby's recent MadSci Genetics Network answer to a question, very similar to yours, about changing DNA segments. As Dr. Silby indicated, although it might not be altogether impossible to ever do what you suggest, it would certainly be very, very difficult.
With regards to your question more specifically, I'd like to clarify the concept of gene inactivation & reactivation. Your question seems to imply that humans carry genes capable of forming other creatures, such as spiders, upon 'reactivation', which is not the case. Although humans have evolved from lower forms of life, we are not direct descendents of all lower forms of life, such as spiders, nor do we still carry the complete genetic material necessary to re-form those creatures from which we have evolved.
When organisms evolve, their genes gradually change to optimize their survivability. For example, at some point in the evolution of 4-limbed vertebrates (amphibians, reptiles, birds, & mammals), the forelimbs of ancestral birds began to develop into wings, while those of the ancestral amphibians, reptiles, & mammals did not. So there are no inactive 'wing genes' present in amphibians, reptiles, or mammals. In your example, spiders evolved in a completely different branch of the evolutionary tree from humans. So most of the genes expressed to form a spider would not be carried by humans. Finally, even if a gene was present in one of our direct ancestors, such as apes, it would have most likely been gradually changed during evolution so that the original gene was no longer present. For example, apes have much longer & stronger forelimbs than humans. But humans do not carry any long-strong forelimb genes capable of 'reactivation'. Our forelimb genes evolved from those long-strong forelimb genes & essentially replaced them. It is possible for some genes to become inactive, rather than gradually changed, during the course of evolution. However, genes that are no longer necessary for survival tend to accumulate mutations that eventually prevent them from producing functional proteins. These are sometimes identified as pseudogenes. So any 'inactive genes' would generally be incapable of functional 'reactivation'.
Whether genes from an actual spider could be transferred into a human to form a 'spider-creature' or not, as also suggested by the previous question about changing DNA segments, is more difficult to answer. Although it is very unlikely that one organism could ever actually be transformed into another, expression of individual genes from foreign organisms is already possible in human laboratory cells, & human genes have been transferred to & expressed in mice. However, the number of genes & complexity of their expression necessary to create just one aspect of a 'foreign creature', such as a spider's exoskeleton, is enormous. Furthermore, many traits are simply not functionally transferable from one type of organism to another. For example, even if humans could be genetically programmed to grow wings, our bony skeletons are much too heavy to be capable of flight.
This is probably a much longer explanation than you
expected, but you asked a very thought-provoking question that's
edge of scientific plausibility,
Jeff Buzby, Ph.D.
CHOC Research Institute
MadSci Genetics Network
Try the links in the MadSci Library for more information on Genetics.