Down syndrome: what it is, causes, diagnosis and more

Table of Contents

Down syndrome is not a disease, but a genetic alteration caused by the presence of an extra chromosome (the chromosomes with the structures containing the DNA.

Down syndrome: what it is, causes, diagnosis and more

What is Down syndrome

Down syndrome is not a disease, but a genetic alteration caused by the presence of an extra chromosome (the chromosomes with the structures containing DNA, which is the main constituent of the genetic material of living beings) or a part of it.

The cells of the human body have 46 chromosomes distributed in 23 pairs. People with Down syndrome have three chromosomes in the 21st pair instead of the usual two. This is why it is also known as trisomy 21.

Causes

The ultimate cause is unknown. It is a chromosomal alteration that occurs during gestation. The specific problem arises in the replication of chromosome 21 and can originate in the following ways:

Trisomy 21

The most common type of Down syndrome is called trisomy 21, which is the result of a genetic error that occurs very early in the cellular reproduction process.

Chromosomal translocation

Chromosomal translocation occurs in very rare cases. During the process of cell division, a chromosome 21 breaks and one of these fragments (or the entire chromosome) is abnormally attached to another chromosome pair, usually chromosome 14.

Mosaicism or mosaic trisomy

Once the egg is fertilized, in the subsequent division process the genetic material does not separate correctly, so that one of the daughter cells has three chromosomes in its 21st pair and the other has only one.

There is no specific cause that determines why these alterations of chromosome 21 occur, but there are risk factors that indicate an increased likelihood of developing it.

Diagnosis

Down syndrome can be diagnosed prenatally or after birth.

Prenatal diagnosis

There are different ways of detecting Down syndrome during pregnancy, with varying degrees of reliability. Prenatal testing can be for suspicion (screening) or confirmation.

The most important of the latter is amniocentesis, which is usually performed only if there is a history of genetic alterations or if the screening tests give a high risk of the fetus having the alteration.

Diagnosis after birth

It is made by clinical examination and subsequent confirmation with karyotyping, i.e. analysis of the size, shape and number of chromosomes.

Further information

History

Down syndrome is named after John Langdon Haydon Down, a British physician who first described the clinical features in 1866, although he did not know the causes. Trisomy 21 was not discovered until 1958 by the French geneticist Jérôme Lejeune.