Preimplantation Genetic Diagnosis - PGD

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Preimplantation genetic diagnosis (PGD) evaluates known carriers of specific single-gene defects, such as cystic fibrosis, and parents whose offspring are at increased risk for selected chromosomal abnormalities such as Trisomy 21 (Down’s syndrome), Turner syndrome, and certain unbalanced translocations.

Typically, the couples in need of these techniques are not infertile. In fact, in most cases, the couple has had an affected child and is seeking the opportunity to diminish the risk of having another child with significant health compromise and early death.

PGD utilizes in vitro fertilization where multiple eggs are matured and retrieved. The oocytes are then inseminated with a single sperm (ICSI), and grown in culture until the 6-8 cell stage. At this point, the embryo is biopsied with the removal of 1-2 cells. This process does not damage the cells remaining within the fertilized egg.

The isolated cells are then evaluated for the specific genetic defect anticipated. Two main techniques are used for the determination: Polymerase Chain Reaction (PCR) and Fluorescent in Situ Hybridization (FISH).

• In PCR, multiple copies of the gene of interest are made by a process of amplification. This amplification process allows the identification of very small amounts of material in order to make the diagnosis.
   
• FISH allows the laboratory to actually count the number of chromosomes within the isolated cell. This technique is utilized primarily for expected abnormalities in chromosome number (e.g. trisomy – three copies of – 21 or Down’s syndrome) or translocations (defects in the structure of the chromosome).

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