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Down syndrome is a genetic disorder that encompasses a set of cognitive and physical symptoms caused by the presence of an additional copy of chromosome 21 and is characterized by a variety of dysmorphic features, congenital malformations, health problems and medical conditions.
It is the most frequent form of intellectual disabilities caused by a chromosomal aberration.

Named after the British physician John Langdon Down who first fully described the syndrome as a disorder in 1866, some aspects of the condition were already outlined earlier by French psychiatrists Jean-Étienne Dominique Esquirol in 1838 and Édouard Séquin in 1844.

It will be later in 1959 that French researchers identified the genetic cause of Down syndrome as an extra copy of chromosome 21.

For the first time in the history of genetics, a link was established between a mental disability and chromosomal anomalies. 

What are the genetic changes related to Down syndrome?

Chromosomes contain all the individual hereditary characteristics unique to each person. Those characteristics are encoded in the individual’s DNA.
The smallest unit of genetic information is called a gene.
Genes are the blueprint for human growth and development; they control how each cell develops and functions.

Each human cell normally contains 23 pairs of chromosomes, one set from each parent, for a total of 46 chromosomes.

Chromosomal abnormalities usually happen as a result of an error in cell division called nondisjunction: chromosomes fail to equally separate during meiosis.
Meiosis is the name used to describe a cell division stage at which the egg and sperm undergo when they are developing.
Normally, meiosis causes a halving of chromosomal material, so that each parent gives 23 chromosomes to a pregnancy.

When fertilization occurs, the normal 46 total number of chromosomes results in the fetus. If meiosis does not occur properly, an egg or sperm could end up with too many chromosomes or not enough.

These cell division abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental disabilities associated with the Down syndrome.

Besides, three genetic variations can cause Down syndrome: Trisomy 21, Mosaic Down syndrome or translocation Down syndrome.

The most common form of Down syndrome is known as Trisomy 21.
Indeed, about 95% of cases result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies.

Researchers admit that having extra copies of genes on chromosome 21 disrupts the course of normal development, causing the characteristic features of Down syndrome and an increased risk of health affections such as hearing loss, obstructive sleep apnea, ear infections, eye diseases, heart defects and digestive abnormalities.

Additionally, a small percentage of children with Down syndrome develop cancer of blood forming cells: leukemia.

All of these factors can further affect the life of the child and of the family.

What is chromosome 21?

Chromosome 21 is the smallest human chromosome, spanning about 48 million base pairs, the building blocks of DNA, and representing 1,5 to 2% of the total DNA in cells.

In 2000, researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome.
Chromosome 21 was the second human chromosome to be fully sequenced.

Genetic research seeks to identify the number of genes on each chromosome. However, the estimated number can vary depending on how scientists use different approaches to predict it.
Chromosome 21 likely contains 200 to 300 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.

Down syndrome occurs in about 1 in 1000 newborns. Although women of any age can have a child with Down syndrome, the chance increases as a woman gets older.

Prenatal testing aims to give families and doctors the earliest and most complete information on eventual chromosomal disorders in the fetus.
Invasive diagnostics such as Chorionic Villus sampling (CVS) or Amniocentesis provide an accurate diagnostic but generate anxiety in the expecting mother and have a risk of miscarriage.

Currently, the whole genome shotgun sequencing method and advanced bioinformatics ensure excellent reliability on every screened disorder. Cell-free fetal DNA testing is an authentic revolutionary approach, safe and precise.

For further information visit Tranquility, the most complete non-invasive cell-free fetal DNA screening for Trisomy disorders and sexual aneuploidies.