Preimplantation Genetic Diagnosis (PGD) in Genetic Screening
In vitro fertilization (IVF) revolutionized infertility treatment and opened the door for many new and exciting technologies. These technologies include intracytoplasmic sperm injection (ICSI) which gave “infertile males” new hope for creating genetically related children and preimplantation genetic diagnosis (PGD). 
Preimplantation genetic diagnosis (PGD) provides a means to screen embryos for specific abnormalities prior to uterine transfer in an IVF cycle. PGD can screen for an abnormal number of chromosomes (aneuploidy), chromosomes that are broken or translocated, and specific genetic diseases. PGD is extremely accurate in identifying male and female embryos making gender selection, or family balancing, a scientific reality. Many couples want to experience the joy of raising children of both genders.
In addition to family balancing, sex selection has a major role in preventing sex linked genetic diseases. These are disorders that can only present in a child of a particular sex. For example, hemophilia A and B are X (female chromosome) linked recessive and can only occur in males (XY). If a woman has the gene for hemophilia, after PGD only female embryos can be transferred to the uterus thus eliminating the chance for hemophilia in the resultant child.
The risk of genetic disorders is directly related to female age. Women over 35 years of age are at higher risk for chromosomal abnormalities, which can lead to early miscarriage or conditions such as Down’s syndrome.
In the PGD procedure, one or two cells are removed from the embryo and genetically evaluated in a procedure known as a blastomere biopsy. The biopsy involves removing a small sample of the cells DNA. Initially, the cells have not differentiated and a biopsy can be taken, usually on day 3, without damaging the embryo. Results of the biopsy are typically available within 48 hours. This means that the embryos transferred have the opportunity to develop to the blastocyst stage.
Embryos are examined using fluorescent in situ hybridization (FISH) and/or the polymerase chain reaction (PCR). FISH involves labeling chromosomes with fluorescent tagged probes and examining them under a special microscope. These tags allow gender determination, identification of missing or extra chromosomes, and translocations.
PCR creates multiple copies of specific identified areas of the chromosome and is used to screen for single gene defects. When the genetic disease is present in the parent, a copy of a segment of his/her gene can be used to make a genetic probe. This probe will “identify” the genetic disease if it is present in the embryo.
At the current time, PGD can be used to screen for more than 120 specific genetic diseases such as Tay Sachs, cystic fibrosis, Fanconi anemia, fragile X syndrome, hemophilia A, and others. As more genetic diseases are characterized they are added to the list of those which can be screened.
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