Chromosomal Diseases: Definition and Types of Existing Alterations in Chromosomal Structure

They represent the existing alterations in the chromosome structure of the organism’s cells, which generates various pathologies which will depend on the affected or mutated chromosome.

Human beings have a total of 46 chromosomes, of which 23 come from the mother and the other 23 from the father and are located in the nucleus of cells that carry DNA.

However, when the chromosomal structure is being formed and some abnormality or mutation occurs, diseases of this nature originate. Among the most common are alterations in the number of sex chromosomes.

When these abnormalities in chromosome structure occur, they can occur:

  • Miscarriages.
  • Mental retardation
  • Congenital malformations.

Types of chromosomal diseases

The most common chromosomal diseases are:

Number abnormalities: this failure is not compatible with life. Generally, the unborn dies before birth. However, there are three that can develop and allow the unborn to be born and survive after birth:

 Trisomy 21 (Down syndrome)

It is the busiest in the world because 1 in 650 people suffer from it. People with this disease have the following physical characteristics:


  • Skull dysmorphia, usually small and round.
  • Nape flat, short or large.
  • Excess skin on the nape.
  • Short nose and, in some cases, flattened.
  • Smallmouth.
  • Wide tongue.
  • Mental retardation.
  • Visceral malformations.
  • Cardiac and digestive dysfunction in some cases.

Trisomy 18 (Edwards syndrome)

This chromosomal disease is severe; even in most cases, those who suffer from it die within weeks of birth. The essential characteristics of this anomaly are the following:

  • Microcephaly.
  • Hypotonia .
  • Dolicocephaly.
  • Pointy ears.
  • Smallmouth.
  • Deformation in the feet.
  • Severe encephalopathy
  • Heart deficiencies
  • Chronic disease in the kidneys and digestive system.

It is not a common syndrome because 1 in 8,000 newborns suffers from it, with females being more likely to have this condition than males.

Trisomy 13 (Patau syndrome)

It is a rare condition to suffer from since most of the unborn with this condition die before birth. Among the characteristics of this disease are:

  • Craniofacial malformations.
  • Vast and small nose.
  • Front bulging and bulging.
  • Soft ears.
  • Mouth with an outward protrusion like the mouths of wolves.
  • Deformations of the hands and feet.
  • Sometimes cebocephaly.
  • Heart, digestive system, or brain conditions.

Newborns with this chromosomal disease do not usually last more than 130 days due to all their body’s deficiencies.

Chromosome 5 cord deletion (cat’s meow syndrome)

It affects 1 in 50,000 people, and its characteristics include:

  • A high-pitched, sad cry is similar to that of a cat.
  • Microcephaly.
  • The excessive separation between eyes.
  • Moon face.
  • Small ears.

Deletion of the short of chromosome 4 (Wolf-Hirschhorn syndrome)

It affects the female sex more than the male. Among the characteristics of this condition are:

  • Mental retardation.
  • Facial malformations.
  • Visceral affections.
  • Seizures
  • High and grooved forehead.
  • Straight nose.
  • Physical development problems.
  • Wide frown.
  • Small chin.
  • Short upper lip.
  • Congenital heart disease.

Chromosomal abnormalities

Chromosomal abnormalities can be grouped into two categories:

  • Numerical.
  • Structural.

Numerical abnormalities occur in the fertilization process when the nucleus of cells composed of the 23 chromosomal pairs is being formed.

Among them are the homogeneous ones generated just in the meiotic division process or, in some cases, during the fertilization process.

Meiosis or meiotic division is cell reproduction that gives rise to gametes. That is why when a failure occurs, the chromosomal structure is affected, which will spread in the zygote’s cells when fertilization occurs.

Anomalies known as polyploidies can also be generated, characterized by generating more chromosomes than usual in the fertilization process.

This mutation in the chromosomes causes spontaneous abortion in the gestation cycle or the individual’s death shortly after birth.

The most common polyploidy is triploidy, which consists of three additional sets of chromosomes, giving a total of 69 chromosomes.

Another anomaly is aneuploidy, where only one pair of chromosomes is affected, developing one or more or no one; that is, a trisomy or monosomy can be generated.

Trisomies where the fetus does not die in the gestation process, not after the birth of the unborn, is on chromosome 21, 13, and 18. However, the most common aneuploidies are those that affect the sex chromosomes.

In the gestation process, supernumerary chromosomes can develop, which are rare.

They are characterized by having a useless presence in the chromosomal structure. However, they generate negative consequences in the development of the fetus, either due to the appearance of malformations or an emotional nature.

Other chromosomal disorders

Mosaics: it occurs in fertilization when the cells are already developed by mitosis and begin forming the fetus. This abnormality is not found in all cells, only those generated during mitosis.

Chimeras: it is generated when a formed zygote unites with an ovum, thereby generating double female genetic material, which leads to half of the cells having an additional X chromosome. Among the diseases that this anomaly generates is sexual ambiguity.

Structure anomalies

The mutations or abnormalities in the structure are less frequent than those existing in the numerical ones. Among them are:

Delegations: consists of the partial loss of a part of the chromosomal material. As for example, there is the elimination of the collar of chromosome 5. This failure gives rise to the cat syndrome, there is also the failure in chromosome 4, which produces the Wolf-Hirschhorn syndrome.

Translocations: occurs when genetic material is exchanged between two or even more chromosomes, which can be classified into:

  • Reciprocal.
  • Robertsonian.
  • Complex

Simple reciprocal translocations arise between two chromosomes that are not homologous. According to some studies, there is an average of 1 person in 625 who is a carrier of a translocation.

Robertsonian translocations are generated between two acrocentric chromosomes, this means that they have the center shifted towards the short arms, therefore, the short arms are shorter and the long arms are longer.

Complex translocations occur between three chromosomes. Men who suffer from this anomaly tend to have infertility problems, whereas women can be reproductive without difficulty.


They are an alteration that mainly affects a single chromosome when it breaks and inverts its arms.

The so-called pericentral occurs when the inverted fragment includes the centromere. This anomaly occurs in 1 out of every 1000 people.

When the inversion is called paracentral, it is because the inverted fragment does not include the centromere and the individual with this condition does not necessarily have to have any disease.


This occurs when a segment of a chromosome is inserted into another chromosome, it can also occur when it is inserted into another region of the same chromosome.

People or individuals who present this anomaly do not necessarily manifest themselves in deformations, diseases or genetic failures, however, they are more prone to their descendants acquiring an anomaly in their chromosomes.


This is generated when one of the arms of the chromosome breaks off and disappears in division.

The most frequent are in the long arm of the X chromosome, which would result in the suffering of Turner syndrome.