Isochromosome: What is it? Origin, Nomenclature and Associated Diseases

It is an abnormal metacentric chromosome formed by duplicating one arm of a normal chromosome with deletion of the other arm.

It occurs when duplication is performed in the transverse plane rather than in the vertical plane.

In this case, the isochromosome is eccentric.

One of the two centromeres of a dicentric isochromosome generally does not function, so the chromosome is usually secreted during cell division.

When there is a complete loss of one arm of a chromosome, this is equivalent to the monosomy of this arm, and when this arm is “replaced” by the duplication of the other arm, this is equivalent to the trisomy of the other arm.


In an isochromosome, the mirror image consists of two copies of a short arm or a long arm. Eccentric (two centromeres) and monocentric (one centromere) formations have been found.

A U-type chain change between sister chromatids in the short arm (dicentric) or centromere mid-division (monocentric) is probably the mechanism for isochromosome formation.


Currently, it has been determined that an isochromosome can be formed through two mechanisms:

  1. It is formed from an exchange of one of the arms of a chromosome with the homologous chromosome or the sister chromatid. Both arms of the metacentric chromosome are genetically identical. by an isochromatic breakdown and the fusion of the daughter chromatids above the centromere.
  2. It can arise in meiosis II when an error occurs in the centromere division. There is a transverse division of this instead of the longitudinal division, and it is the least common form of formation of an isochromosome.


The vocabulary of the isochromosomes is carried out by placing I, followed by a double punctuation mark, the parenthesis, which contains the number of the chromosome where the anomaly occurred and the arm involved, examples such as I (17q) or i (17) (q10 ), indicate that the duplication was performed in arm q and arm p was lost.

Associated diseases

Isochromosomes are associated with diseases that are often X-linked and acrocentric chromosomes.

X chromosome

The Turner syndrome is caused by the total or partial absence of a second normal sexual chromosome in individuals of the female sex.

One of the causes is due to duplication (isochromosome) of the long arm of an X (46, X, I (Xq)).

Chromosome 12

Pallister-Killian syndrome is characterized cytogenetically by mosaic tetrasomy of chromosome 12p.

Routine prenatal diagnosis remains complicated due to difficulties in discriminating between supernumerary isochromosome 12p and duplication 21q and the variable level of mosaicism present.

This disorder occurs during the formation of reproductive cells and usually occurs in the mother.

This is a condition characterized by soft childhood muscle tone ( hypotonia ), intellectual disability, epilepsy, and distinctive facial features such as:

  • The broad forehead.
  • Presence of little hair on the temple.
  • Large spaces between the eyes.
  • Presence of epicanthic fold.
  • Flat nose

They can also have vision problems and deafness.

Chromosome 14

It has also been observed that the isochromosome of the long arm of 14 is associated with solid tumors and malignant hematological tumors.

Chromosome 17

Isochromosome 17q is the most common isochromosome in cancer.

It plays a vital role in tumor development and progression and hematologic malignancies such as chronic myeloid leukemia.

Isochromosome 17q has a poor prognosis; it is the most common chromosomal abnormality in primitive neuroectodermal tumors and medulloblastoma.

Chromosome 18

The occurrence of tetrasomy 18p is the result of an extra abnormal chromosome, called isochromosome 18p, in each cell.

Isochromosome 18p has two identical p arms. Usually, the cells of organisms have two copies of each chromosome, and each one is inherited from each parent.

In people where the 18p tetrasomy is present, the cells have two regular copies of chromosome 18 and one isochromosome plus 18p.

The additional genetic material found on the isochromosome usually disrupts the ordinary course of development of the individual and causes the typical features of this disorder.

This condition usually causes feeding difficulties in infancy, developmental delay, and intellectual disability that is often mild to moderate but severe, changes in muscle tone, distinctive facial features, and other physical congenital disabilities.

However, the signs and symptoms vary between affected people.

Chromosome 20

Interstitial deletion of the long arm of chromosome 20, as the only abnormality, is commonly seen in myeloid malignancies, including myeloproliferative disorder, myelodysplastic syndrome, and acute myeloid leukemia.