The terms "human genetic modification" or "human
genetic engineering" mean changing genes in a living human
There are two types of genetic modification. Somatic
modifications involve adding genes to cells other than egg
or sperm cells. If you had a lung disease caused by a defective
gene, scientists might be able to add a healthy gene to your
lung cells and alleviate the disease. The new gene would not
be passed to any children you may have.
Germline modifications (also called "inheritable
genetic modifications") would change genes in eggs, sperm,
or very early embryos. The modified genes would appear not only
in any children that resulted from such procedures, but in all
succeeding generations. This application is by far the more
consequential, because it would open the door to the alteration
of the human species.
[Note: The term somatic comes from the Greek
soma for "body." The term germline refers
to the germ or germinal cells, i.e., the eggs and sperm.]
Genes are strings of chemicals that help create the
proteins that make up the body. They are found in long coiled
chains called chromosomes located in the nuclei of the cells
of the body:
Genetic modification occurs by inserting genes into living
cells. The desired gene is attached to a viral vector,
which has the ability to carry the gene across the cell membrane.
Proposals for inheritable genetic modification in humans combine
techniques involving in vitro fertilization (IVF), gene
transfer, stem cells and cloning:
As shown above, germline modification would begin by using
IVF to create a single-cell embryo, or zygote. This embryo would
develop for a few days to the blastocyst stage, at which point
embryonic stem cells would be removed. These stem cells would
be altered by adding genes using viral vectors. Colonies of
altered stem cells would be grown and tested for successful
incorporation of the new genes. Cloning techniques would be
used to transfer a successfully modified stem cell nucleus into
an enucleated egg cell. This "constructed embryo"
would then be implanted into a woman's uterus and brought to
term. The child born would be a genetically modified human.
(Images courtesy of the Association of Reproductive Health
Last modified June 1, 2006