What is Down Syndrome?

Down Syndrome is a genetic condition that occurs when the individual has three copies (total or partial) of chromosome number 21 instead of two, a situation that causes particular characteristics in the appearance, development and health of the person.


Most individuals have in each of their cells a nucleus containing 23 pairs of chromosomes (23 chromosomes provided by the father and 23 by the mother) that translate to 46 chromosomes, which attached to the sex chromosomes XX or XY would classify reg people in two genetic groups: 46 XX (women) and 46 XY (men).

However, sometimes in pair 21 there are three copies of chromosomes, which would also result in 23 pairs of chromosomes, but a final count of 47 XX (for women) and 47 XY (for men).

Features and causes

This extra copy in par 21, produces certain characteristics in the individual such as hypotonia (reduced muscle tone), short stature, almond eyes, flattened nose, sometimes a single palmar fold, among other phenotypic characteristics. Individuals with this condition may also suffer from congenital heart disease, among other health conditions, such as kidney disease, poor intestinal formation, neurological problems.

However, these conditions do not occur in all cases, being people with down syndrome quite healthy individuals. Similarly, there is some intellectual commitment, ranging from mild to moderate and profound, which creates another type of intelligence and level of communication. However, like the rest of humans, there is no individual with down syndrome equal to another, each having its own unique characteristics.

This genetic situation, also known medically as Trisomy 21, is a human condition that has always been present, being mentioned in literature and embodied in art. However, it was until the late 19th century that a British scientist and physician named John Langdon Down made the first scientific description. Despite not being the only one or the first, he did describe it as a separate condition.

However, it had to be expected until 1959, when Jéréme Lejune, a French physician, managed to identify that the characteristics of down syndrome (named after John Langdon Down) were produced by a genetic situation. More recently, in 2000, another major scientific breakthrough occurred when a group of researchers were able to identify and catalog each of the 329 genes on chromosome 21, opening the door to a greater understanding of the conditions caused by an extra copy of this one.

Despite these advances, Science has not yet been able to decipher why this condition occurs. At the moment there is no explanation that can link Down Syndrome to any environmental condition or any specific parent activity before or during pregnancy. So in most cases it is considered a random event, also because all of us are prone to having sons or daughters with this condition, which is also one of the most common chromosomal situations, having an index of 1 every 691 births.

Types of down syndrome

On the other hand, unlike conditions such as autism, down syndrome has no degree, since an individual has or does not have this condition, that is, he has or does not have within his par 21 three chromosome copies. What varies from individual to individual, as in other human beings, are physical and intellectual characteristics.

However, three types of down syndrome can be differentiated, which differ according to the origin of the extra chromosome copy and its occurrence in all or only a few cells. In this sense, we have the free Trisomy 21, also known as Non-Disjunction, which occurs when an alteration occurs in cell division that results in three copies of chromosome 21.

This can occur in the gametes of the parents (both in the egg and sperm) or during embryonic development. This type of trisomia 21 accounts for 95% of cases of down syndrome.

Likewise, mosaicism can occur, a situation that occurs when non-disjunction occurs but does not occur in all cells, causing the individual to have some cells with 46 XX or XY count, and others with 47XX or XY. The person with mosaicism (present in only 1% of cases) may have milder phenotypic characteristics of the syndrome, although most geneticists point out that this does not imply a decrease in the characteristics of this condition.

Similarly, there is translocation trisomy, which occurs in 4% of cases, when a part (or cane) of the chromosome is broken during cell division, joining another chromosome, with the number 14 being the most common. In this case the chromosome count will give 46 XX or XY, but the individual will have an extra copy of chromosome 21.

These three types of genetic situations occur randomly, i.e. in most cases, down syndrome isn´t given by an inherited factor that can be identified in one of the parents. Only 1% is given because a parent has a translocated chromosome, which originates in the future baby the extra copy of chromosome 21.

In fact, all people, regardless of their social and economic status, are likely to have a child with this condition, which every day becomes less limiting, thanks to the understanding and integration of society, which has allowed us to know the infinite capacities of people with this condition, who today study, work, live as a couple and reap great successes in the professional, sports and artistic areas.

Image source: labec.com

What is Down Syndrome?
Source: curiosities  
August 22, 2019


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