Preimplantation Genetic Diagnosis: Fluorescence in Situ Hybridization

Preimplantation genetic diagnosis, more commonly known as PGD, is a technique used to detect possible genetic defects in embryos produced in the in vitro fertilization procedure. The FISH, short for Fluorescence in Situ Hybridization, is one of the three PGD techniques available.

FISH in PGD

Fluorescence in Situ Hybridization is the most frequently used technique of PGD, and can analyze the chromosomes that make up the embryo. If any abnormalities are detected, the baby is likely to be born with a genetic condition, which can be avoided if FISH is applied.

The FISH technique will test the interphase chromosomes, which will be carefully analyzed and hybridized with DNA samples. The test requires just a few cell samples from the embryo. The procedure is performed after the embryo is formed and prior to implanting it in the mother's uterus.

The chromosomes will be analyzed in segments, so that the analysis can be accurate. Both the X and the Y chromosomes can be tested. Several FISH tests can be performed on the same embryo, to test for all the known abnormalities.

The Fluorescence in Situ Hybridization test can be used in couples that resort to IVF if they have a history of genetic disorders in the family or are considered carriers of defective DNA, which can be transmitted to the baby. However, even if there are no risks in the partners, the embryo may be tested for the sake of safety and to ensure that the pregnancy will be viable and the baby healthy.

Uses of FISH

Fluorescence in Situ Hybridization can be used to determine if there are any chromosomal disorders that can lead to the formation of genetic diseases such as:

  • X-linked disorders
  • Down syndrome
  • Hemophilia A
  • Huntington's

In addition, the Fluorescence in Situ Hybridization test can also determine if there are types of aneuploides present, which increase the chances of a successful implantation and a viable pregnancy out of the resulted embryo. At the same time, the presence of certain aneuploides can indicate high chances of spontaneous abortion during the pregnancy.

Success Rates of FISH

The Fluorescence in Situ Hybridization procedure is known to give results that are over 90 percent accurate. The test may fail to detect chromosomal abnormalities in 5 to 10 percent of the tested embryos. However, there are no means to find out how many of the discarded embryos are actually viable.

Shortcomings of FISH

The Fluorescence in Situ Hybridization technique may have some disadvantages. The embryos tested may have a high mosaic rate and may present a chromosomal fault one day, and test negative for the same defect the following day. Some scientists believe that just a few cells from the embryo don't carry significant information on the composition of the entire embryo. The embryos that are found faulty may actually be viable, and these are discarded. The Fluorescence in Situ Hybridization is considered a technique with limitations which are not present in the haplotyping technique.

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