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Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human somatic cell having 45 or 47 chromosomes instead of the usual 46. [ 1 ] [ 2 ] It does not include a difference of one or more complete sets of chromosomes .
3b—A genetic test is run on each embryo for a given trait and the results are matched with the embryos. 4—The embryos without the desired trait are identified and discarded. 5—The remaining embryos are allowed to grow to the point that they can be implanted. 6a—The embryos with the desired trait are implanted.
Another test that may be performed is a chromosomal microarray, which can detect chromosomal imbalances, such as small- and large-scale deletions and duplications. [8] Chromosomal microarray can be performed on cultured or uncultured fetal tissue, with results available as early as 3 days. [5]
A karyotype of an individual with trisomy 21, showing three copies of chromosome 21.. An abnormal number of chromosomes is known as aneuploidy, and occurs when an individual is either missing a chromosome from a pair (resulting in monosomy) or has more than two chromosomes of a pair (trisomy, tetrasomy, etc.).
Normal cells make errors in chromosome segregation in 1% of cell divisions, whereas cells with CIN make these errors approximately 20% of cell divisions. Because aneuploidy is a common feature in tumour cells, the presence of aneuploidy in cells does not necessarily mean CIN is present; a high rate of errors is definitive of CIN. [6]
The results of the blood test are then combined with the NT ultrasound measurements, maternal age, and gestational age of the fetus to yield a risk score for Down syndrome, trisomy 18, and trisomy 13. First Trimester Combined Test has a sensitivity (i.e. detection rate for abnormalities) of 82–87% and a false-positive rate of around 5%. [73] [74]
Non-Invasive Prenatal Testing, or NIPT, is a simple blood draw that screens for chromosomal abnormalities of a fetus while still in utero. When this screening was first performed it was used to determine the sex of a fetus, now it is also used to find aneuploidies in fetal DNA. [6]
Trisomy X, like other aneuploidy disorders, is caused by a process called nondisjunction. Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis , the process that produces gametes (eggs or sperm), and result in gametes with too many or too few chromosomes. [ 34 ]