Fluorescent In Situ Hybridization

Currently fluorescent in situ hybridization (FISH), the most popular PGD technique, is used to count the number of certain chromosomes and observe how they’re arranged. Through fluorescent in situ hybridization (FISH), scientists can determine if an embryo has two X chromosomes (a girl) or an X and Y chromosome (a boy).

Each chromosome has a specific pattern of DNA near its center, which is identical in nearly all of us. When fluorescent probes are placed on a cell, they attach to the specific region that they match, if that region is present. In other words, after a cell is exposed to the probes, fluorescent light is applied and the cell can then be “read”. For example, if the probe for the X chromosome is the color red then the technician sees two red signals, which means that two X chromosomes are present and the embryo is a female. The Y chromosome lights up with another color.

The overall accuracy of fluorescent in situ hybridization (FISH) is about 90%, although it is far higher when looking solely at the X and Y chromosomes (sex chromosomes) for gender.

Typically, when a five-probe fluorescent in situ hybridization (FISH) is employed, looking at chromosome X, Y, 13, 18, and 21, which identifies the gender of the embryos and screening for Down syndrome and the other chromosomal abnormalities that could result in a live-born infant.

The fluorescent in situ hybridization (FISH) technique has several drawbacks. First, because fluorescent in situ hybridization (FISH) cannot examine all twenty-three chromosome pairs, it’s not very useful as an embryo-screening device. Secondly, fluorescent in situ hybridization (FISH) cannot look for single-gene disorders, such as cystic fibrosis, and it does not preclude the need for otherwise indicated prenatal testing such as amniocentesis.


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