Index
The cells of the human body contain 46 total chromosomes, grouped into 23 pairs of chromosomes.
Half of the chromosomes come from the mother, and the other half comes from the father. The first 22 pairs are called autosomes and the 23rd pair with the so-called sex chromosomes: X, Y.
Women usually have two X chromosomes, and men generally have one X and one Y chromosome in each cell.
All the information the body needs to grow, develop, and reproduce comes from the chromosomes.
Each chromosome contains thousands of genes, which produce proteins that direct development, growth, and chemical reactions in the body.
Chromosomal abnormalities can occur as an accident when the egg or sperm are formed or during the early stages of the fetus’ development.
The age of the mother and certain environmental factors can play a role in the occurrence of genetic errors .
Prenatal evaluations and tests can be done to examine the chromosomes of the fetus and detect some, but not all types of chromosome abnormalities.
Chromosomal abnormalities can have many different effects, depending on the specific abnormality.
Chromosome abnormalities can also cause miscarriage, disease, or growth or development problems.
Types of chromosomal abnormalities
The human body has 20,000 to 25,000 different genes.
Genes are found on chromosomes, which are rod-shaped structures in the middle of every cell in the body.
When a gene or chromosome is altered, it can cause health problems in the body.
There are many types of chromosomal abnormalities, which can be classified as numerical or structural:
Numerical abnormalities
Numerical alterations or abnormalities originate mainly due to nondisjunction, when paired chromosomes or sister chromatids do not separate in the usual way.
The result can be complete chromosomes that are missing or are extra for the normal pair.
Euploidia
Organisms whose number of chromosomes are multiples of a basic number (n).
- Monoploid: 1 set of chromosomes.
- Triploid: 3 sets of chromosomes (3n), a monoploid gamete (n) plus a diploid gamete (2n).
- Tetraploid: 4 complements of chromosomes (4n), somatic doubling of the number of chromosomes.
- Polyploid: more than 2n chromosomes.
Triploidy and tetraploidy are lethal and represent 5% of the causes related to spontaneous abortions.
Aneuploidia
It refers to the variations that occur apart from the chromosomal complement.
- Monosomic: they have lost a single pair of chromosomes. Genomic formula 2n-1.
- Trisomic: they are diploids that have an extra chromosome, Genomic Formula 2n + 1.
- Double trisomics: two chromosomes are each represented in triplicate, the double trisomic can be symbolized as 2n + 1 + 1.
- Nulisomic: You have lost a chromosome pair. The result is commonly lethal for diploid formula 2n-2.
Structural abnormalities
They occur when a part of an additional, individual chromosome is missing, changes to another chromosome, or is turned upside down.
Structural chromosomal abnormalities are the result of breakage and incorrect reentry of chromosome segments.
A range of structural chromosomal abnormalities results in disease.
Structural rearrangements are defined as:
Balanced structural rearrangements
They occur if the complete chromosome set is still present, albeit rearranged.
Balanced rearrangements include inverted or translocated chromosomal regions.
Since the full complement of DNA material is still present, balanced chromosome rearrangements can go unnoticed because they may not lead to disease.
A disease can arise as a result of a balanced rearrangement if chromosome breaks occur in one gene, resulting in an absent or non-functional protein, or if the fusion of chromosome segments results in a hybrid of two genes, producing a new protein product whose function is detrimental to the cell.
Unbalanced structural rearrangements
Possible structural aberrations in chromosomes are diverse and include:
Deletions
Where part of a chromosome is completely lost.
Deletions can affect any chromosome, being of any size and in any part of it.
This loss of genetic material can have devastating effects on the individual, when the removed material contains vital instructions for the body.
The severity of this depends on how much of the chromosome has been removed and where the deletion is.
Disorders such as developmental delay, health or learning problems can occur, the best known aberrations include Wolf-Hirschhorn syndrome, caused by a deletion in chromosome 4, and Jacobsen syndrome, caused by a deletion in the end of long arm of chromosome 11.
Duplications
Where part of a chromosome is copied, causing an addition of genetic material.
An exemplary human disease is Charcot-Marie disease type 1A, which can be caused by a duplication of genes on chromosome 17.
When a chromosome has duplicated a part of itself, there is too much chromosome material present.
This extra chromosome material can mean that there are too many instructions for the body to process, and this can lead to learning disabilities, developmental delay, and / or health problems in a child.
Translocaciones
Where part of a chromosome is removed and moved to another chromosome.
Translocations can be subdivided into two types:
Reciprocal translocation, where two chromosomes exchange segments and Robertson translocation, where an entire chromosome joins another at its centromere, forming a dicentric chromosome.
In humans, Robertsonian translocations only occur between acrocentric chromosomes 13, 14, 15, 21 and 22, any Robertsonian translocation involving chromosome 21 can cause Down syndrome in the offspring of that individual.
The effect of these translocations depends on whether any chromosomal material has been lost or gained.
Translocations can lead to miscarriage or the birth of children with symptoms including physical disabilities and learning disabilities.
Investments
In which a portion of the chromosome is removed, turned over, and reinserted from back to front.
It comes in two types: paracentric and pericentric. Investments generally do not cause any health problems for the person making the investment.
Rings
Where a portion of the chromosome breaks and forms an independent circular structure, with or without the loss or gain of genetic material.
This usually happens when both ends of the same chromosome are removed. The remaining ends of the chromosome are “sticky” and stick together to form a ring.
Isocromosomas
Where one chromosomal arm is lost and replaced by an exact mirror image of the remaining arm.
Inheritance of chromosomal abnormalities
The way a gene is inherited can help determine the risk of a gene in a current or future pregnancy. This risk increases if:
- In the progeny there is a son with a genetic disorder.
- There is a family history of a genetic disorder.
- Either parent has a chromosomal abnormality.
- When the ultrasound is performed in the uterus, the baby has some abnormality.
Families at risk for genetic diseases should report health histories, to understand chromosomal abnormalities and to help members who are planning a pregnancy to show the risks of certain problems.
And go to specialists who provide genetic counseling to understand its effects and the forms of prevention or treatment.
Genetic testing
Each parent’s DNA may need to be checked. This is to learn about some patterns of genetic inheritance.
Usually when there is a risk of chromosomal alterations, prenatal diagnoses can be carried out in the uterus, before birth to detect any chromosomal alterations.
The tests may include:
- Cytogenetic tests: this test consists of an evaluation through cell cultures of all the chromosomes of the individual to detect alterations.
- Biochemical tests: in this test, through biochemical reactions, it is possible to know if there is a genetic mutation in any type of protein that causes some type of disease. The tests are used to quantify the activity of a protein and the activity, size or quantity of a protein can be indirectly known.
- Molecular testing: DNA testing can be done on any sample, even very small tissue samples. These tests are very challenging as some diseases can be linked to many genetic abnormalities.
During pregnancy tests such as:
- Ultrasound: This test uses sound waves to watch a baby grow in the womb.
- Chorionic villus test : This test uses a sample of tissues around the baby to look for problems.
- Amniocentesis: This procedure is performed by obtaining a sample of the amniotic fluid for analysis.
