A chromosome is a condensed structure of DNA (deoxyribonucleic acid) present in cells that appears in constant number in each plant or animal species. Much of the genetic information is stored in the chromosomes.

The shape of the chromosomes varies according to the cell stage. While in interphase they are impossible to visualize with a light microscope, in meiosis or mitosis they acquire the more familiar X structure.

The specific appearance of each chromosome is given by the centromere, a region that, when stained, is less stained than the other parts of the structure. The ends of the chromosome, meanwhile, are called telomeres. Each chromosome has a particular position of the centromere and a certain length, and from each of them there is another identical one. This means that homologous chromosomes form pairs (two chromosomes are the same).

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Homologous chromosomes have the same genes located at the same places (known as loci ). In those organisms that reproduce sexually, one of the pairs of chromosomes is contributed by the mother and the other by the father. Thus, each diploid organism has two copies of each gene.

In the case of the human being, the constant number of chromosomes of the species is 46, since each individual has 23 pairs of homologous chromosomes. Of these 23 pairs, 22 correspond to somatic chromosomes and the rest to sex chromosomes ( XX in women and XY in men).

When an alteration of these constants occurs, a genetic disorder is generated. Down syndrome, for example, develops when there is an extra copy of chromosome 21 : in that case, there are three chromosomes instead of two.

The so-called chromosomal diseases arise from abnormalities in the chromosomes, being the alteration of the number of sex chromosomes the most common of all. The symptoms and consequences of these disorders are very serious, since they usually cause miscarriages, mental retardation and congenital malformations.

Broadly speaking, it is possible to group chromosome anomalies into two large groups: those of number and those of structure.

Chromosomal number abnormalities

During sexual reproduction, the father and the mother each contribute 23 chromosomes that unite and give rise to the 46 that we must have in our genetic material to enjoy good health and undergo adequate development. Depending on the point of gestation at which the anomaly occurs, the characteristics are different.

If the abnormalities take place during the meiotic division or fertilization, then they are called homogeneous. This division is also known as meiosis and it is the process by which gametes are created; if something unusual happens at this time, then the abnormality will affect the rest of the cells of the zygote during fertilization.

Chromosomal abnormalities of structure

This type of anomaly occurs less frequently than number ones; however, since they can go unnoticed due to a lack of symptoms or visible effects, the percentage of the population it affects is not known with great precision.

One of the most common forms of structural abnormality is deletion, where some chromosomal material is missing, which is associated with Wolf-Hirschhorn syndrome and cat meow syndrome.

Translocations, on the other hand, are the exchange of genetic material that takes place between two or more chromosomes. Inversions are alterations that harm a chromosome when it breaks and its arms are inverted ; it is pericentral if a fragment including the centromere is inverted, and paracentral in the opposite case.