Long “chain” of DNA wrapped around itself until it assumes the typical shape of a stick. Each cell in the human body contains 46 chromosomes organized into 23 distinct pairs (Cariotype). For each pair, we receive a chromosome from the father and one from the mother.
Sex chromosomes are defined by X and Y, an individual who has the pair XX is a female, an individual who has the pair XY is male.
CHROMOSOME
and CARIOTYPE
DNA
it is the genetic material contained in all the cells of all living organisms (DNA is the acronym of Acid Desossi ribo Nucleico).
Its constituents (called nucleic acids) follow one after another forming a long chain and the sequence in which they appear determines, as a code, the set of all the information necessary for life.
GENES
The genes correspond to portions of the genome located in precise positions within the sequence (DNA or, more rarely, of RNA) and contain all the information necessary for the production of a protein. They are contained and organized within the chromosomes, present in all the cells of an organism.
Human cells contain all 23 pairs of chromosomes, with the sole exception of gametes, which have a single copy of each chromosome.
ANEUPLOIDIE , TRISOMIE E MONOSOMIE
The human chromosomal arrangement consists of 46 chromosomes organized into 23 distinct pairs, the aneuploidies are anomalies in the number of chromosomes.
These abnormalities may cause the presence of an additional chromosome (trisomy) or lack of a chromosome (monosomy)
MICRODELETIONS
The microdeletions occur the loss of a tract of a chromosome and, consequently, of the genes located on that chromosomal fragment.
These alterations cause syndromes of clinical importance depending on the chromosome involved, the region involved and the size of the lost region. Currently it is not possible to identify this type of chromosomal mutations with the examination of the traditional karyotype on chorionic villi cells or amniotic fluid.
Prenatal diagnosis
(Extracted from Guidelines age.na.s 2013) Prenatal Diagnosis refers to the set of investigations, instrumental and laboratory, through which it is possible to monitor the state of health and well-being of the fetus during the course of pregnancy. The use of prenatal diagnosis techniques is aimed at identifying diseases that affect the fetus on a genetic, infectious, iatrogenic or environmental basis.
At present it is possible to identify only some pathologies of the fetus, but prenatal diagnosis is making great progress and allows us today to look to the future with greater prospects.
The various prenatal diagnosis techniques now developed allow us to identify some multifactorial malformative pathologies, chromosomal anomalies and genetic diseases whose specific defect is known, as well as the presence in the genome of infectious agents.
Prenatal diagnostic techniques can be non-invasive or invasive, depending on whether they include an abortive risk. "Non-invasive methods" are procedures that evaluate the risk of chromosomal abnormalities; they are defined as non-invasive because, based on ultrasound evaluations and or analysis of the maternal blood, they do not pose any risk for the pregnant woman or the fetus. They are aimed at identifying fetal chromosomal pathologies, in order to limit the use of invasive tests. The G test is the most innovative among non-invasive analyzes; it is performed on the DNA of the fetus, ensuring safety and reliability.
The "invasive methods" provide for the taking of tissue samples, by amniocentesis or villocentesis, with an abortive risk between 0.5 and 1%, and subsequent analysis to determine the presence of genetic abnormalities with different techniques depending on the diagnostic needs.
Amniocentesis
Villocentesis
Prenatal diagnostic test which takes place between the 15th and 20th week of gestation, is entrusted to the expert doctor and is associated with a risk of abortion between 0.5 and 1%.
With a needle penetrating the woman’s abdomen and the amniotic sac, a small amount of amniotic fluid is taken (the so-called “waters” that surround the fetus in the uterus during pregnancy).
This operation must be performed by an expert gynecologist using an ultrasound check. The fetal DNA is then isolated from the cells of the amniotic fluid withdrawn.
It is the earliest form of prenatal diagnosis: the examination is performed between the 10th and 12th week of gestation and has a percentage of abortion risk estimated at around 0.5-1%.
By means of a needle penetrating the abdominal and uterine wall, a small amount of chorionic villi is taken (microscopic ramifications that form the outermost part of the placenta). The fetal DNA is then isolated from the cells of the chorionic villi collected.
Frequent questions
Who perform G-test?
The test is performed by Bioscience Genomics Laboratories in Rome (University of Tor Vergata) and in Milan (San Raffaele Hospital).
Who can undergo G-test?
All the pregnancy women from 10-week gestation.
What evaluate the G-test?
The G-test evaluates the risk that the fetus can be affected by the Down Syndrome, Edwards Syndrome or Patau Syndrome, or by trisomy on 9,16,22 chromosomes.
How to do- undergo G-test?
To undergo G-test is necessary an easy maternal blood sample, because inside there is fetal free DNA.
Which is the possibility that in case on low risk, the baby result not affected by trisomy 21, 18 and 13? Higher of 99,99%. This percentage is confirmed by the bigger study on clinical validation never publish in international scientific magazines.
Which is the reliability of G-test?
Which is the probability that in case of low risk, the baby result not affected by chromosomes anomaly of G-test? >99,99%. This percentage is confirmed by the biggest clinical validation study never publish on international scientific magazines.
What is the false positive?
It is talking about false positive when the result indicates the presence of a pathology that is not confirmed by a diagnostic investigation.
The percentage of G-test false positive is less than 0.05%.
Which are the differences related to the screening test not invasive of 1st quarter?
The G-test measuring directly the abundance related chromosomal on the maternal blood, meanwhile the screening test of first quarter are indirect and on statistic base (evaluates the maternal age, ultrasound data and hormonal doses in the mother); it goes with a minor reliability compared to the G-Test.
Why prefer the G-test to the traditional screening test in the first quarter?
The non-invasive prenatal test in the first quarter have a reliability well below; the Bitest, for example, can underline the 21 trisomy also if the fetus is healthy in 5% of cases (is talking about of false-positive) or can get that the fetus is healthy when in real has the Down Syndrome (it is talking about false-negative and it can be verified in the 10-15% of cases).
The G-Test replaces the amniocentesis?
The G-test does not replace the amniocentesis but is a screening test with comparable reliability (> 99%) whose introduction may permit to avoid the 95% of invasive tests resulting with the reduction of abort cases ( limitation of invasive examination solely to case is necessary an in-depth diagnostic analysis) .
