MadSci Network: Genetics

Re: Can the genes of a living organism (as in already born) be altered.

Date: Fri Feb 23 12:29:33 2001
Posted By: Paul Szauter, Staff, Mouse Genome Informatics, The Jackson Laboratory
Area of science: Genetics
ID: 980471714.Ge

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:

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:

See also:

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:

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 

The answer is yes, in a qualified way:
1. If the organism is abnormal because of a mutation in a particular gene, 
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 

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:

I have used a number of genetics terms in this answer. To better understand 
these terms, see:

Thank you for your interesting question. I hope that you enjoy learning more 
about genetics.


Paul Szauter
Mous Genome Informatics

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