The terms "human genetic modification" or "human genetic engineering" mean changing genes in a living human cell.

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 Professionals)

Last modified June 1, 2006