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Karyotyping of fetuses to diagnose aneuploidies has traditionally… Karyotyping of fetuses to diagnose aneuploidies has traditionally depended on extracting fetal cells, either from the amniotic fluid (amniocentesis) or through taking a placental sample (chorionic villus sampling – CVS). Both of these methods pose a risk to the fetus. While the risk for pregnancy loss is relatively low, new techniques have been developed that post no risk to the fetus. These are possible because of the advances in DNA sequencing.This is called non-invasive prenatal testing (NIPT) and depends on cell-free DNA (cfDNA) testing. The mother has small amounts of fetal DNA in her bloodstream. Here’s the basic idea:Take a blood sample from the motherPurify it to only plasma to get rid of the mother’s cells (and thus her DNA) Make millions of copies of the DNA in the plasma using PCR (you read about this in Ch 13.4. We’ll review it again in the last unit.)Sequence the DNA Determine if the “amount of sequence” is more than you’d expect for 2 copies of the chromosome (normal diploid) being present.As reported by Wang et al.”NIPT has been widely applied for screening for trisomies 21, 18, and 13 using cfDNA in maternal plasma [5]. The combined specificity for these autosomal aneuploidies was found to be 99.9% [6].”As of March 2020, this technique has not been as effective in diagnosing sex chromosome aneuploidies (SCA): Turner syndrome, Triple X-syndrome, Klinefleter syndrome, and XYY syndrome. “The positive predictive value (PPV) of NIPT in the present study was 57.6%, which was divided and categorized by individual SCAs as follows: 21.4% for Turner syndrome (45,X), 75.0% for Triple X syndrome (47,XXX), 90.9% for Klinefelter syndrome (47,XXY), and 75.0% for XYY syndrome (47,XYY).The recommendation is to use amniocentesis or CVS to confirm a potential SCA result.Wang Y, Li S, Wang W, Dong Y, Zhang M, Wang X, Yin C. Cell-free DNA screening for sex chromosome aneuploidies by non-invasive prenatal testing in maternal plasma. Mol Cytogenet. 2020 Mar 12;13:10. doi: 10.1186/s13039-020-0478-5. PMID: 32190123; PMCID: PMC7068885. HOw does the cfDNA testing work A By detecting whether there are more copies of sequence than expected in a diploid organism  B By detecting a mutation in the sequence of fetal DNA C by detecting more than one copy of chromosome using fluorescent probes to bind to specific genes (fish technique) D  By taking a sample of the placenta and sequencing the DNA Biology Science Genetics BIOL 2401

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