The 13 types of chromosomal abnormalities (structural and numerical)
In the biological world, it all comes down to genetics. Human beings (and any other living being on Earth) are nothing more than "sacks" of genes. Richard Dawkins, in his famous book "The Selfish Gene", spoke of how we are simple carriers of DNA units that seek to perpetuate their existence.
On both the positives and negatives, we are dominated by our genetics. The 30,000 genes of our genome are those that determine the physiology of each and every one of our cells and, ultimately, of our organism as a whole.
And our DNA, along with other proteins, are organized into what we know as chromosomes, which are each of the highly organized structures that contain most of our genetic material. In the case of human beings, who are diploid organisms, we have 23 pairs of chromosomes. And it is, therefore, in these 46 chromosomes present in each of our cells that hides the genetic information that makes us who we are.
Still, genetics are not perfect. Y it is possible that these chromosomes suffer alterations in their structure or in their number that, depending on the affectation, derive in more or less serious problems for the integrity of the person suffering from said anomaly. In today's article, then, we will investigate how chromosomal abnormalities or mutations are classified.
What is a chromosomal abnormality?
Chromosomal abnormalities or chromosomal mutations are alterations in the structure of chromosomes or changes in the normal number of these. These are, therefore, genetic defects that affect chromosomes and which, depending on the degree of impact on the genetic material, will have different effects at the physiological level.
It is estimated that 1 in 200 babies are born with some type of chromosomal abnormality, which can lead to diseases, growth problems, intellectual disabilities and it is even possible that the pregnancy cannot run its course and is interrupted in the form of abortion spontaneous.
Chromosomal abnormalities are alterations that can occur by chance when the egg or sperm is formed or when the fetus is beginning to develop. However, certain factors (advanced age of the mother or certain environmental influences) can increase the risk that these genetic accidents in the structure or number of chromosomes occur.
Chromosomes are each of the highly organized structures present in the nucleus of our cells. that, being made up of DNA and proteins that allow its cohesion (its most visible form and what we understand as a chromosome occurs during division, when the cell has to pack its DNA), they contain the genetic information of a living being.
In the case of humans, who are diploid organisms, we have 23 pairs (46 in total) of homologous chromosomes (they have the same genes located in the same place as their partner but with different genetic information). Half of them come from our father and the other half, from our mother. Our 30,000 genes are packaged on these chromosomes. The first 22 pairs are the autosomes, while the last pair (23) are the sexual ones: men are XY and women are XX.
It is not surprising, then, having seen that these chromosomes contain all the genetic information that determines our physiology, that abnormalities in their structure or alterations in the total chromosome number, lead to potentially serious complications.
How are chromosomal abnormalities classified?
As a general rule, chromosomal abnormalities or mutations are divided into two large groups depending on whether the alteration consists of a change in their structure or a change in the total number of chromosomes, thus having structural chromosomal abnormalities or numerical chromosomal abnormalities, respectively. Let's see the particularities and subtypes within each one of them.
1. Structural chromosomal abnormalities
Structural chromosomal abnormalities are those alterations in which, due to different genetic reasons or protein expression (due to a previous genetic mutation in the nucleotide sequence of a given gene), the structure of a chromosome is damaged. The integrity of the chromosome is lost and, depending on which (and how many) genes are involved, the consequences will be more or less serious. Many of these anomalies end in spontaneous abortions, as a functional organism cannot develop. Let's see what types exist.
1.1. Chromosomal deletions
Chromosomal deletions are abnormalities that consist of a more or less large part of a chromosome is lost. These deletions or losses of chromosomal portions can occur anywhere on any chromosome and, depending on how much (and which) genes are lost, the effects will be more or less severe. An example of this type of chromosome abnormality is Cri du Chat syndrome, a rare disease that develops from the partial deletion of the short arm of chromosome 5.
1.2. Chromosome duplications
Chromosomal duplications are abnormalities that consist of a segment of a chromosome is repeated. Instead of having two copies of the same genetic segment (by pairs of homologous chromosomes), the person has three. An example of this type of abnormality is Fragile X syndrome, which develops from a partial duplication of the end of the long arm of the X sex chromosome.
1.3. Chromosomal inversions
Chromosomal inversions are abnormalities consisting of a change in direction of a genetic segment within a chromosome. The chromosome "breaks" in two places and the resulting DNA segment is reinserted but in the reverse direction, altering the way genes are transcribed into proteins.
1.4. Balanced chromosomal translocations
Balanced chromosomal translocations are abnormalities that consist of a genetic segment of a chromosome moves and inserts into another chromosome without there being a loss or gain of total DNA. In the end, the genetic functionality is maintained, the genes are simply on another chromosome.
1.5. Chromosomal unbalanced translocations
Unbalanced chromosomal translocations are anomalies that consist, again, in that a genetic segment of a chromosome moves and is inserted into another chromosome, although in this case, there is an alteration (due to loss or gain of DNA) in said segment . Therefore, genetic functionality is in jeopardy.
1.5. Chromosomal insertions
Chromosomal insertions are abnormalities that consist of a segment of DNA from a chromosome has been transferred to an unusual position within the same chromosome or another. Again, if there is neither loss nor gain of DNA, the person will not suffer from any syndrome, just as with balanced translocations.
1.6. Ring chromosomes
Ring chromosomes are abnormalities that consist in that, due to a deletion of both arms of a chromosome, the "broken" ends of the same end up joining to thus give the shape of a ring. This structure in itself is not dangerous, the problem is given by the background deletion.
1.7. Uniparental dysomias
Uniparental dysomias are abnormalities that consist of both chromosomes of the same pair come from the same parent. For example, chromosome 15 from the mother is duplicated and chromosome 15 from the father is absent. It is not a numerical anomaly since the total number of chromosomes is still 46, what happens is that in a given pair, the two chromosomes come from a parent. Prader-Willi Syndrome is an example of this anomaly and follows the same pattern that we have taught, being a maternal uniparental disomy of chromosome 15.
Isochromosomes are abnormalities that consist of one chromosome has lost one of the arms and the other has doubled. This happens when the division of the centromere (the structure that joins the sister chromatids) occurs in a transverse plane rather than vertical.
1.9. Diccentric chromosomes
Diccentric chromosomes are abnormalities consisting of a fusion of two segments of a chromosome, which results in the appearance of two centromeres. Instead of having only one centromere, happens to have two due to this abnormal fusion of chromosomal portions.
2. Numerical chromosomal abnormalities
In numerical chromosome abnormalities, the structural integrity of the chromosomes is not impaired, but there is an incorrect number of chromosomes in the genome. That is, there are more (or fewer) chromosomes than there should be, and therefore the person does not have 23 pairs of chromosomes. A person with a total number of chromosomes other than 46 suffers from a chromosome number abnormality. Let's see how they are classified.
Monosomies are abnormalities that consist of the loss of a chromosome. That is, in one of the pairs of chromosomes, the person does not have two copies, but only one. The resulting individual thus has a total of 45 chromosomes instead of 46. Turner syndrome is an example of monosomy on the X chromosome.
Trisomies are abnormalities that consist of the gain of a chromosome. That is, in one of the pairs of chromosomes, the person does not have two copies, but three. The resulting individual, then, has a total of 47 chromosomes instead of 46. Down syndrome is the clearest example and it is a trisomy of chromosome 21.
2.3. Autosomal aneuploidies
Autosomal aneuploidies are all those numerical abnormalities that occur in the autosomes, that is, in the non-sex chromosomes, which are the first 22 pairs. These are not always viable and those that are, produce alterations in the phenotype. Down syndrome, Edwards syndrome, Patau syndrome… These are examples of autosomal aneuploidies.
2.4. Sexual aneuploidies
On the other hand, sexual aneuploidies are all those numerical abnormalities that occur on the sex chromosomes, that is to say, in pair number 23. These anomalies are usually always viable and as examples we have Turner Syndrome, hemophilia, Triple X Syndrome, Klinefelter Syndrome, and so on.