|MadSci Network: Genetics|
The short answer to your question is yes, it is possible to alter the genes of a fully developed organism, including an adult human. To fully answer your question, I will start with some background information. Since you ask about people, I will confine this discussion to work on mammals, although a great deal has been learned about genes using model organisms such as fruit flies and nematodes. Mammals start development as a single cell (the fertilized egg). This cell divides many times, producing essentially identical cells. As the number of cells increase, they begin to differentiate, that is, show changes in appearance that are grossly apparent, such as becoming nerve cells, skin cells, muscle cells, and so on. As cells differentiate, they express different groups of genes, meaning that different genes are transcribed to make different collections of proteins in different cell types. All of the cells still have all the genes that the original cell started out with, even if they have differentiated. The fertilized egg first develops into a ball of cells that does not resemble an adult organism at all. As development proceeds, the embryo gradually assumes the shape of the adult animal. You can see a set of drawing of this process in the mouse at: http:// genex.hgu.mrc.ac.uk/Databases/Anatomy/Diagrams/ There are a large number of genes that function during embryonic development, but which are not expressed in later embryonic stages or in the adult at all. These genes act to direct the development of the body plan, and mutations in these genes can alter development, producing animals with developmental defects (examples would be cleft lip and palate and spina bifida) or animals that cannot complete development because of profound developmental defects (absence of the heart, for example). It is possible to correct some genetic defects by introducing a normal gene into some of the cells of an adult animal. Not all genetic defects can be corrected in this way. For example, if an animal has a developmental defect resulting from a mutation in a gene that functions during embryonic development, adding that gene to some of the cells in the adult will have no effect, because development is over. On the other hand, some genetic defects are good targets for 'gene therapy', which has been successfully carried out in humans. Here is a good introduction to gene therapy: http:// www.med.upenn.edu/ihgt/info/whatisgt.html See also: http://www.accessexcellence.org/AB/IWT/ Gene_Therapy_Overview.html The most successful human gene therapy has been against 'Severe Combined Immune Deficiency' or SCID. This is a rare genetic disorder resulting in the complete absence of an immune system; some affected patients have lived in sterile 'bubbles' for years before dying of infections. One form of this disease is the result of the absence of a gene called 'interleukin-2 receptor, gamma chain', which encodes a cell surface receptor protein necessary for the development of lymphocytes (white blood cells). You can see a press account of successful gene therapy for this disease at: http://www.abcnews.go.com/sections/living/DailyNews/ genetherapy000427.html It was possible to treat this disease because the gene for the interleukin-2 receptor component must be expressed in bone marrow. Because it is possible to draw a sample of a patient's bone marrow, add the gene to some of the cells, and return the marrow, it is possible to treat this disease with gene therapy. This is an example of an 'autologous transplant', in which the recipient is also the donor. Bone marrow transplants using tissue-matched donors are widely carried out as a treatment for leukemia. Returning to your question: 'would there be any significant effect on the organism's physiology and anatomy?' The answer is yes, in a qualified way: 1. If the organism is abnormal because of a mutation in a particular gene, AND 2. If that gene is normally expressed in adult cells, AND 3. If those cells are successfully transformed with the normal gene, AND 4. If only some of the cells need the normal gene in order to restore normal function, THEN: The answer is yes. As for changes in anatomy, the disease muscular dystrophy results in a wasting of muscle tissue. Experimental gene therapy for muscular dystrophy appears to increase muscle mass to normal, so there can be effects on anatomy. At this time, there is not much interest in adding genes to normal adult organisms in an attempt to change them. Many desirable characteristics (strength, for example) are the result of complex interactions of many genes during development, so it is not likely that we could increase our strength through gene therapy rather than through athletic training. I should also point out that changing the genes of a one-celled embryo, to which you refer, will change the genes in all of the cells of the resulting animal, and these changes can be inherited by the offspring of that animal. This is different from gene therapy, in which only some of the cells have an added gene. For a discussion of making transgenic embryos, see: /cgi-bin/circR?/posts/ archives/980908691.Ge.r.html I have used a number of genetics terms in this answer. To better understand these terms, see: http:// www.informatics.jax.org/userdocs/glossary.shtml Thank you for your interesting question. I hope that you enjoy learning more about genetics. Yours, Paul Szauter Mous Genome Informatics
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