Chromosomal Abnormalities: What are they? How do they occur? Numerical Anomalies, Structural Abnormalities and Their Types

Almost all cells in the human body have 23 pairs of chromosomes, which make up a total of 46 chromosomes.

Of the chromosomes that cells contain, half come from the mother, and the other half come from the father.

The first 22 pairs of chromosomes are autosomes (non-sex) and the 23rd pair consists of two sex chromosomes, which are X and Y. Females have two X chromosomes, and males have one X and one Y chromosome on each. cell.

Chromosomes contain all the information that the body needs for their formation, since they contain thousands of genes, which produce the proteins responsible for carrying out the chemical processes of the body.

Historically, scientists have used a staining technique that colors chromosomes in a banding pattern. These banding patterns make each of our individual chromosomes easier to identify, like a map.

A set of chromosomes, as seen under a microscope, is known as a karyotype, and any deviation from the normal karyotype is known as a chromosomal abnormality. While some chromosomal abnormalities are harmless, some are associated with clinical disorders.

Numerical anomalies

They happen when there is an irregularity with respect to the number of chromosomes. The most severe chromosomal disorders are caused by the loss or gain of entire chromosomes, which can affect hundreds, or even thousands, of genes and are usually fatal.

Some numerical abnormalities support development to term, either because the chromosome is small and / or contains relatively few genes, or because there is a natural mechanism present to help adjust the gene dosage.

Structural abnormalities

They happen when large sections of DNA are missing or added to a chromosome. Structural abnormalities can take several forms:

  • Deletion: it is a mutation that causes the loss of a part of the chromosome.
  • Duplication: is a mutation that causes part of the chromosome to repeat itself, resulting in additional genetic material.
  • Translocation: is a mutation that causes a portion of a chromosome to move to a different part of the chromosome (intrachromosomal) or to another chromosome (interchromosomal). It can be reciprocal (segments of two different chromosomes are exchanged) or Robertsonian (complete chromosome that joins another).
  • Inversion: it is a mutation that results in a portion of a chromosome that is in the opposite orientation (inverted).
  • Ring: is when part of a chromosome breaks and takes the shape of a circle or ring.

Balanced structural anomalies vs. unbalanced

  • Balanced structural abnormalities involve the reorganization of genetic material, but without overall gain or loss. For example, investments and translocations.
  • Balanced structural abnormalities can have an immediate effect, but the main consequence is the production of eggs or sperm with incomplete or partially duplicated sets of chromosomes.
  • Unbalanced structural abnormalities involve genetic material gained or lost.
  • Even small unbalanced structural abnormalities can affect many genes and consequently have serious effects on the individual.

How do chromosomal abnormalities occur?

Chromosome abnormalities generally occur when there is an error in cell division that results in cells with too few or too many copies of a chromosome.

Most chromosomal abnormalities originate in the egg or sperm (gametes), but some occur during embryonic development or are inherited.

Normally, during the formation of gametes, the two pairs of chromosomes (one from the mother and the other from the father) separate in a process called meiosis. This results in a single copy of each chromosome in the gametes (as opposed to the two copies found in the other cells of the body).

Errors in the separation process lead to the formation of gametes with incomplete sets of chromosomes or all or parts of additional chromosomes. This is known as meiotic nondisjunction.

Some factors that can increase the risk of having chromosomal abnormalities include maternal age (the frequency of meiotic nondisjunction increases with maternal age) and environmental factors or exposure to certain medications.

Depending on the specific anomaly, it can affect an individual’s development in different ways. For example, an extra copy of chromosome 21 causes trisomy 21 (mostly known as Down syndrome).

These abnormalities can also be the cause of problems such as miscarriages , diseases or complications in the growth or development of the embryo or fetus.