Preimplantation genetic diagnosis (PGD)
Preimplantation genetic diagnosis (PGD) tests early-stage embryos produced through in vitro fertilization (IVF) for the presence of a variety of conditions. One cell is extracted from the embryo in its eight-cell stage and analyzed. Embryos free of conditions that would cause serious disease can be implanted in a woman's uterus and allowed to develop into a child.
PGD allows couples at risk of passing on a serious genetic disease to have a child that is fully genetically related to them and that does not carry genes for the disease. It does not involve the manipulation of genes in embryos; rather, it selects among embryos.
To date, PGD has been most widely used to prevent the birth of children with chromosomal diseases such as Down's Syndrome, and with other genetic disorders, including Tay-Sachs disease, cystic fibrosis, sickle cell disease, Huntington's Chorea, and Cooley's anemia.
People who oppose the destruction of human embryos in general are necessarily opposed to PGD; however, for some, PGD is preferable to aborting a fetus affected by a medical condition that has been identified through prenatal screening.
Preimplantation diagnosis and selection is also controversial because it can be considered a eugenic technology. Many disability rights organizations, in particular, have been critical of its uncontrolled use, and point out that the definition of "disease" is to some extent subjective. Most disability rights advocates who criticize PGD and prenatal screening nonetheless support abortion rights. They believe that a woman should be allowed to decide whether or not to have a child at a given time, but not to base this decision on the traits of the particular embryo.
Many people fear that PGD will be used to select a child of a preferred sex. PGD could also be used in attempts to select a future child's cosmetic, behavioral, and other non-disease traits.
However, the genetic laws of independent assortment make it difficult for PGD to be used for any traits that depend on two or more genes. Thus, PGD provides an alternative to germline modification as a way to prevent the births of children with serious genetic diseases, most of which are single-gene disorders, but does not open the door to escalating and species-altering applications.
