Karyotype: the test to detect the presence of chromosomal abnormalities
The karyotype is the chromosomal structure of a person, the set of his chromosomes that contain the DNA, the genetic information. The karyotype examination, therefore, is a test that allows to know the number of chromosomes and their structure, in order to identify any chromosomal abnormalities that may be at the basis of various diseases or syndromes.
A common karyotype is made up of 46 chromosomes: 44 are identical in "men and women, while the other two - the so-called" sex chromosomes "- are different according to sex (XX in women and XY in men). The karyotype examination, carried out in the laboratory, allows the chromosomes contained in a person's cells to be viewed under the microscope: blood cells are generally used, but if the test is carried out on a fetus, the fetal cells present in the amniotic fluid, detected through amniocentesis, or the cells of the chorionic villi through the villocentesis.
The analysis of the chromosomes allows to find possible anomalies in the number of the same (for example if there are trisomies) or in the structure. These alterations are responsible for malformations, congenital defects, abortions or other types of pathological conditions. "is to know about" the subject, but first here is a beautiful video that portrays a loving couple with Down syndrome, also detectable (as we will see) by examining the karyotype:
Karyotype and chromosomal abnormalities
As we have anticipated, the karyotype examination allows to discover any chromosomal abnormalities in the cells through the analysis of the chromosomal number and structure. If these anomalies are found in the fetus they can compromise its growth and development (as happens with Down syndrome). The result of this test can also be used to determine infertility or sterility, or to detect acquired anomalies in patients with leukemia, refractory anemia or specific tumors.
The chromosomal abnormalities that can be identified in cells through karyotype analysis can be of two types: numerical or structural. The numerical anomalies are those in which the number of chromosomes is different from the typical 46 that characterizes the human being. Among these anomalies we have trisomies (when there are three copies of the same chromosome instead of two): trisomy 21 is the one that corresponds to Down syndrome, trisomy 18 to Edwards syndrome, trisomy 13 to Patau syndrome.
If, on the other hand, the presence of one less chromosome is detected, one speaks of monosomies: this is the case of Turner syndrome, in which only the first of the two sex chromosomes is present. Often numerical anomalies in the DNA can lead to miscarriage.
Structural anomalies, on the other hand, concern the structure of the chromosome and can be of different kinds: deletions (when a chromosome fragment is lost), inversions (when a chromosome fragment detaches and inserts itself in the same point but upside down by 180 degrees) , duplications (a fragment is duplicated), translocations (an exchange of a fragment occurs between two different chromosomes). Translocation, when balanced (ie it does not lead to loss or duplication), can cause frequent infertility or miscarriage in a woman.
When is it good to take the exam?
The karyotype examination is prescribed by doctors especially if you are a carrier of a genetic disease, if you have family members (parents or siblings) with chromosomal abnormalities, in case of frequent abortion or sterility, in case of suspected acquired abnormalities following anemia refractory, leukemia, lymphoma, myeloma or other cancers.
The test is usually done to babies after birth if syndromes such as Down's are suspected. In adulthood, on the other hand, couples who have problems with infertility or frequent abortion are subjected to the examination. Pregnant women can also test the fetus to identify any anomalies, but it is recommended only if the risk exists, for example if the mother has an advanced age or if the parents themselves carry chromosomal alterations.
How is it done?
The karyotype examination consists in taking the cells, which are then cultured in the laboratory. Here the cells are divided in order to better observe the chromosomes, which appear more evident during this procedure.
The cells, at the right moment, are fixed on a slide and stained. The specialized operator will then be able to identify any anomalies in the number or in the structure. The examination is always carried out on an empty stomach and must be prescribed by a specialist doctor.
How to read the results?
If the chromosomal arrangement does not reveal any anomalies, the result of the examination should be equal to 46, XX for a woman and 46, XY for a man. Each different result will be considered anomalous, but it does not necessarily mean that you have a genetic disease: the anomaly needs to be studied and explained by experts in cytogenetic analysis.
There are, of course, some characteristic congenital anomalies, but the severity of the pathologies can vary greatly from person to person. Furthermore, the examination of the karyotype does not allow to identify the mutations responsible for genetic diseases such as cystic fibrosis or haemophilia or thalassemia, which need to be studied with specific molecular biology techniques.
Some of the chromosomal pathologies that can be identified by examining the karyotype are, in fact, Down syndrome (or trisomy 21), Edwards syndrome (characterized by severe mental retardation), Klinefelter syndrome (due to the presence of a supernumerary X chromosome in the sex chromosomes), Patau syndrome (with the supernumerary chromosome 13) and Chronic Myeloid Leukemia, characterized by the Philadelphia chromosome that arises from the translocation of genetic material between chromosome 9 and chromosome 22.
For further scientific information on the karyotype, you can consult the Humanitas website.
