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Pre-implantation Genetic Diagnosis (PGD)
Scientists at the Jones Institute for Reproductive Medicine at Eastern Virginia Medical School developed the first pre-implantation genetic diagnosis (PGD) program. This program helps parents whose children-to-be are at increased risk for selected genetic diseases, such as cystic fibrosis, Tay-Sachs, muscular dystrophy and thalassemia.
A normal pre-embryonic cell will have 23 chromosomes from the mother and 23 chromosomes from the father, for a total of 46 chromosomes. Commonly, the early dividing cells will not equally divide the chromosomal complement. This occurs more often as the age of the mother increases, and is one of the most important reasons why fertility declines with increasing age. This is also why the incidence of chromosomal and congenital defects increases with age.
By using the process of fluorescent in situ hybridization (FISH), physicians can count the number of specific chromosomes - generally the chromosomes more commonly involved in abnormalities, such as chromosome 21, chromosome 18, or the number of X and Y chromosomes. The FISH technique can also be used to evaluate specific chromosome structural rearrangements known as translocations. When a person carries a balanced translocation, his or her children are at risk of having an unbalanced translocation, resulting in either extra or missing pieces of the involved chromosomes.
Another important application of FISH is to determine the status of pre-embryos prior to embryo transfer. Through the PGD program, embryos shown unaffected with the studied gene are diagnosed and transferred, enabling couples at a genetic risk to have healthy children. Additionally, by only transferring embryos without specific chromosomal defects, the total number of embryos transferred can be reduced, thus reducing the risk of triplet or quadruplet pregnancies.
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