Chromosomal changes

Chromosomal changes can affect the number or structure of chromosomes. Down syndrome, for example, is a numerical chromosomal abnormality.

Chromosomal changes or chromosomal mutations occur in the number or structure of chromosomes and can lead to genetic problems and even death in humans.

The so-called numerical chromosome changes are those that cause changes in the number of normal chromosomes for the species. These changes can be aneuploidy or euploidy. Structural chromosomal alterations, in turn, are alterations in the structure of the chromosome and can be deletions, duplications, inversions or translocations.

Summary of chromosomal changes

  • Chromosomal changes are changes in the number or structure of chromosomes.
  • Numerical chromosomal changes are those that affect the number of chromosomes in the individual.
  • Numerical changes can be classified into euploidies and aneuploidies.
  • Structural chromosomal changes affect the structure of a chromosome and are the result of rearrangements and breaks in these structures.
  • Structural chromosomal changes include deletion, duplication, inversion, and translocation.


What are chromosomal changes?

Chromosomal changes are changes that occur in the number or structure of chromosomes . Alterations in the chromosomes can lead to changes in the phenotype of the carrier individual and even lead to death.

Types of chromosomal changes

→ Numerical chromosomal alterations

Numerical chromosome changes are those that affect the normal number of chromosomes of a given species . One of the causes of these chromosomal alterations is the so-called non-disjunction.

In meiosis , the normal thing is that the chromosomes are distributed to the daughter cells without any error and in an equitable way. In cases of nondisjunction , what is observed is that homologous chromosomes do not separate properly in meiosis I or that sister chromatids fail to separate correctly during meiosis II.

As in nondisjunction the chromosomes are not distributed properly, a formed gamete receives two chromosomes of the same type, and the other gamete does not receive any chromosome of the affected type. When these abnormal gametes fuse with normal gametes, there is a zygote formed with an abnormal number of chromosomes for that species.

In the aforementioned case, there is aneuploidy, as only one type of chromosome is affected. In aneuploidies, there is an increase or decrease of one or more chromosomes, but not of the entire chromosome set. There are situations, however, in which the entire chromosome set is affected, an alteration known as euploidy. Euploidies can occur, for example, due to nondisjunction of all chromosomes or failures in the division of the zygote.

◾ Aneuploidy

Aneuploidies , as highlighted earlier, are characterized by either the increase or decrease of one or more chromosomes without compromising all pairs . Aneuploidies can compromise both autosomal and sex chromosomes. The most common cases are:

  • monosomy

It occurs when there is absence of a chromosome when compared to the normal chromosome set. In this case, the karyotype is represented by 2n-1. An example of monosomy is Turner Syndrome , in which only one X chromosome is present.

  • trisomy

When the chromosome is present in triplicate , we say that there is a trisomy. Trisomy is represented by the 2n+1 karyotype. Chromosome 21 trisomy or Down syndrome is the best known in our species.

◾ Euploidy

It occurs when there is a change in the complete group of chromosomes in the cells . To determine whether a ploidy is normal or an alteration, we must consider the number of chromosomes in that species.

In the case of animals , for example, gametes are haploid (they have only one set of chromosomes represented by n), while the rest of the cells are diploid (2n). Depending on the number of chromosomes presented in the species, there are some specific types of euploidy.

  • Monoploidy or haploidy

In changes known as monoploidy or haploidy, there is only one basic set of chromosomes .

  • polyploidy

In polyploidy, such as triploidy and tetraploidy, in turn, there is an increase in chromosome sets . In the case of triploidy, the individual is 3n, that is, has three chromosomal groups. In the case of tetraploidy, the individual is 4n and has four chromosome groups. In humans, polyploidy leads to miscarriage . However, there are several cases of polyploidy in nature considered normal, being, for example, a common condition in plants.


→ Structural chromosomal alterations

Structural chromosome changes , as the name suggests, are related to the structure of the chromosome They can occur due to failures in meiosis or to harmful agents such as radiation .

◾ Deletion

Deletion occurs when a piece of the chromosome is lost. In this way, the chromosome is missing some genes. An example of a disorder caused by a deletion is Cri-du-chat syndrome (“cat meow”), which is the result of a deletion on chromosome 5. The syndrome was so named because the patient’s cry resembles that of a cat.

◾ Duplication

Duplication occurs when a chromosome has an extra segment, that is, a portion of the chromosome is duplicated.

◾ Inversion

In inversion, as the name suggests, the chromosome has a portion with an inverted orientation.

◾ Translocation

Translocation occurs when a fragment of the chromosome separates and binds to a nonhomologous chromosome.



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