Because of its rapid turnaround time, fluorescent in situ hybridization (FISH) is commonly used in hospitals and laboratories as an aneuploid screen of cells retrieved from amniocentesis and chorionic villus sampling (CVS). Chromosomes \(13,18,21, X,\) and \(Y\) (see Chapter 8 ) are typically screened for aneuploidy in this way. Explain how FISH might be accomplished using amniotic or CVS samples and why the above chromosomes have been chosen for screening.

Answer: FISH is a molecular cytogenetic technique used to detect and localize specific DNA sequences on chromosomes by using fluorescent probes that bind at high sequence complementarity. To use FISH as an aneuploid screen for chromosomes 13, 18, 21, X, and Y, samples containing cells are collected from a pregnant woman and prepared for hybridization with fluorescent-labeled probes. These probes bind to their respective target chromosomes, and the samples are then visualized under a fluorescence microscope. The number of fluorescence signals per cell indicates the copy number of the chromosomes, and an abnormal number suggests aneuploidy. These specific chromosomes are chosen for screening because they are associated with relatively common and well-characterized genetic conditions, such as Patau syndrome, Edwards syndrome, Down syndrome, and various sex chromosome-related disorders. Identifying potential genetic risks during prenatal testing can help inform decision-making for parents and healthcare providers.