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What is the cause of trisomy 21?

Trisomy 21 is a genetic disease that affects the whole person. It results from a chromosomal anomaly: normally, the man has 46 chromosomes (cf lexicon) organized in 23 pairs. In trisomy 21, chromosome 21 is in three copies instead of two, bringing the total number of chromosomes to 47. Chromosome 21 is the smallest of the chromosomes, with approximately 255 genes.

It is the presence of this extra chromosome that throws the whole genome and therefore the organism off balance.

This genetic disease can be compared to a computer with too many applications open at the same time. The computer is not broken; it is still running…but in slow motion, because it cannot manage all this information at the same time. In order to get it working again at a good speed, you have to close some windows.

Scientific research aims to silence the excess information expressed in these patients (the "over-expression of chromosome 21"), by inhibiting some of the over-expressed genes from the extra chromosome 21.

Is trisomy 21 a disease?

If we define a disease as an alteration of the health, of the functions of living beings, then trisomy 21 is indeed a disease since the presence of a supernumerary chromosome 21 leads to organic and functional alterations whose association and evolution are characteristic.

On the other hand, the term disease implies the possibility of medical intervention and treatment, which tends to give a hopeful vision of Down Syndrome. This is why Jérôme Lejeune used the term "disease of the intelligence".

Is trisomy 21 a congenital disease?

A congenital anomaly is an anomaly present at birth whatever its cause (hereditary, viral, toxic…).

Trisomy 21 is a constitutional disorder, since it is associated with a particular constitution, in this case a chromosome 21 present in 3 copies.

This constitutional affection can be associated :

- congenital anomalies, malformative or not: heart disease, digestive malformation, congenital cataract…

- medical problems occurring during life: thyroid anomaly, epilepsy, keratoconus…

- to an intellectual deficiency which is constant.

All these manifestations linked to trisomy 21 are not present at birth, they can be revealed later on. This is what justifies the regular monitoring of patients by doctors who know which anomalies to look for at each age of life.

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