Pre-implantation Genetic Diagosis (PGD) is a relatively new technique that allows the embryologist and physician to evaluate parts of the developing embryos genome. As the name implies, certain genetic diseases can be diagnosed prior to implantation of the embryo into the uterus. PGD will provide a general insight into each embryo and show if any common chromosome abnormalities are found. Pre-implantation Genetic Screening or PGS can evaluate the embryos even further on a single gene level to determine if abnormalities are present. This is especially useful if both partners are a carrier for a particular type of disease and are trying to avoid transferring an effected embryo.
PGD employs two separate procedures known as fluorescent in situ hybridization (FISH) and the polymerase chain reaction (PCR). Using FISH, the chromosomes are labeled with fluorescent tags and certain abnormalities such as broken or translocated chromosomes, or an abnormal number of chromosomes can be identified. These abnormalities are more prevalent in older women which is one reason the miscarriage rate is higher.
FISH can also determine the sex of the embryo with a very high degree of accuracy. PCR is used to make multiple copies of DNA for testing for subtle disorders on the gene level. Based on a known structure of DNA, a copy of the matching strand called a DNA probe is created using PCR. This strand will then bind, or hybridize, to that specific matching gene within a parent’s chromosomes.
Once the mutation in the parent is characterized, the probes can be used to identify the mutation in the embryos prior to transfer. PGD is used for gender selection and for identifying embryos that may have a sex linked genetic disease. Many genetic diseases only occur if they are present in an embryo of a particular sex.
For example, hemophilia is a sex linked disease that is transferred from the mother to a male child but not a female. A woman with hemophilia undergoing IVF with PGD would only transfer female embryos to her uterus. There are many genetic diseases that are sex linked. The number of genetic diseases that can be identified using PCR continues to increase as researchers characterize the disorders.