What is Embryo Selection?

Embryo selection is different from germline therapy! Embryo Selection is a process of taking eggs from a female, fertilizing them in vitro (ie: outside of the woman's body), and then letting the fertilized egg divide to the 8 or 16-cell blastomere stage. After this, the egg is tested for the presence of certain undesirable genes or defects. (For more information on how one zygote from the same parents might possess an undesirable gene but another one will not, see our Introduction.) If such defects are present, the embryo is discarded. Usually, several cultures are done at a time, and the process continues until a defectless embryo can be found. (2)

How is Embryo Selection Used Today?

Embryo selection is used only to test for single-gene defects like cystic fibrosis. If a family knows that they are at risk for a disease like cystic fibrosis, Huntington's disease, Sickle Cell Anemia, or Alzheimer's, they may opt to use embryo selection in order to have a child that will not be at risk for these diseases. (11) For a specific case study of how Embryo selection is being used today, see this page on Alzheimer's Disease.

At the current time, embryo selection is also playing a key role in nurturing the development of certain endangered species that are bred in captivity. For many endangered species, the number of animals in captivity from which sperm or eggs can be taken is very limited. This limited gene pool often leads to defects that are associated with inbreeding, and the health of the animal and sometimes its entire species is compromised by these defective animals. Embryo selection can help with this issue by allowing breeders to select for certain favorable genes while eliminating embryos that possess unfavorable genetic traits. (16)

What are the Drawbacks and Strengths of Embryo Selection?

Because embryo selection involves merely testing a blastomere for certain genetic characteristics as opposed to actually altering the genetic makeup of the cell, it is considered to be much less complicated than germline gene therapy. However, embryo selection, like in vitro fertilization, is still a very lengthy and expensive process. Many trials may be necessary before a blastomere can be found that does not possess the undesirable genes. If multiple trials are required to obtain a viable fertilized egg, the process will involve the discard of embryos that do not possess the desired genetic makeup. This may be an ethical issue for prospective parents. (2)

In addition, it is very important to realize that embryo selection is only truly useful for single-gene defects. When one gets into a discussion of testing embryos for specific traits, like height, and attempting to select an embryo because it will produce a taller child, the process becomes dicey. The problem is that many traits, like height, are influenced by more than one gene, it isn't feasible to test for all those particular genes. Mathematical probability equasions have shown that embryo selection, even in an extremely efficient stage of development, will never be of any use for a trait or disease that is controlled by more than five genes. At that point, testing becomes extremely too complex to be of much use.(16)

One should also consider the fact that although an embryo may test negative for a certain disease and will not grow up to develop that disease, when the adult reproduces, he or she could still pass on genes to his or her offspring that contribute to that particular disease. For example, a woman posesses the aa double recessive genotype for a certain recessive disease, so we know that she will develop it later in life. Her husband's genotype is Aa, and thus he will not develop the disease. Fertilized embryos from this couple could be either Aa or aa. Embryo selection will eliminate all the embryos that are aa, so the Aa embryos will grow up healthy, but will still be able to pass on a defective a gene to their offspring. Thus, embryo selection is a one-generation solution, but does not go to the crux of the problem like germline gene therapy does. For more information about genetics, click here!