Down Syndrome Characteristics

Down syndrome, also called Trisomy 21, is a genetic disorder produced by a replication error during cell division, called no disjunction, which causes that in the pair of chromosomes 21, there is an extra chromosome, that is, three instead of the normal two (that is why it is called trisomy).

This syndrome was described in 1866 by the English physician John Langdon Haydon Down, who described its characteristics and symptoms, but its cause until 1958, when the French researcher geneticist Jérôme Lejeune discovered that this syndrome is due to pair 21 containing a chromosome extra.

In 95% of Down syndrome cases, trisomy results from nondisjunction during the gamete meiosis process. This means that as germ cells divide, instead of forming two gametes with 23 chromosomes each, the chromosome 21 does not divide (nondisjunction), resulting in a gamete with 24 chromosomes and a gamete with 22 chromosomes. Of this percentage, 75% occurs during the formation of the ovum, and 25% during the formation of the sperm.

4% occurs due to an imbalance in the translocation with chromosomes 13, 14 or 15. During the formation of the cell, during the replication process, some chromosomes exchange information, which passes from one pair to another; this process is called a translocation. When the translocation fails, a fragment of one of chromosomes 13, 14 or 15 passes to chromosome 21, but is not exchanged, and is added to this pair, causing trisomy.

The remaining 1% is due to a replication error during the embryonic cell mitosis process. When dividing the cell to form the zygote, at some point in cell replication the process fails and an excess copy of one of the chromosomes of pair 21 is produced. This causes that when the cell divides, a normal cell remains, with 46 chromosomes, and an abnormal cell, with 47. This type of Down syndrome, in which there are normal cells and trisomic cells, is called mosaic Down syndrome or mosaicism. In these cases, the symptoms are less marked or some may not appear.

 The risk of birth with Down syndrome is greater as the mother's age increases, with the 1 in 2000 chance in women 25 years of age, 1 in 400 at 35, and 1 in 100 at 40 years.

Physical characteristics of Down syndrome:

• The head is usually smaller than average, with some irregularities in the shape of the skull, an excessive amount of skin on the nape and joints of the cranial bones with certain separation.
• A plump, flattened face, with a flat nose and a slight saddle-shaped indentation; small, irregularly shaped ears and delayed teething.
• The eyes have a slight upward slant, and a fold of skin at the inner corner of the eye (Mongolism); Y Brushfield spots, which are whitish spots on the colored part of the eye.
• The hands are short and wide, the palms have a single fold, unlike the common people which has three folds on the palms, and furthermore its fingers are usually short and somewhat plump.
• Growth retardation and general sluggishness, making motor skills learning difficult.

Psychological and intellectual characteristics of Down syndrome:

• In Down syndrome, children's intellectual development is slow, and they are mentally retarded to varying degrees.
• A deficiency in discernment, especially in language, which makes them vulnerable to not being able to understand or make themselves understood fully.
• Impulsive and somewhat capricious behavior, with a lack of inability to control their emotions.
• They tend to be too distracted, having a very short attention span, so their learning is slow.
• Frustration and anger, especially in cases of mosaicism, seeing their limited capacities both in the physical and psychological fields.
• Development of Mal de Alheimer from the age of 35.

Other diseases that regularly accompany Down syndrome:

• Along with Down syndrome, other conditions develop that affect those who suffer from this congenital disorder, including:
• Heart conditions: intraatrial septal defect, ventricular septal defect, and valve weakness.
• Digestive Conditions: Celiac, esophageal or duodenal stenosis (narrowing), colitis.
• Endocrine disorders: Diabetes, hypothyroidism, and leukemia.
• Musculoskeletal conditions: hip dislocation, neck vertebrae dislocation, especially those that support the head (atlas and axis), short limbs, atony and lack of physical strength.
• Eye conditions: myopia, astigmatism, cataracts.
• Ear disorders: Deafness and balance problems.

It should be noted that the aforementioned characteristics occur more or less frequently in children with Down syndrome and some do not even develop.

• The characteristics that are always present in all cases, although also to varying degrees, are three:
• Mongolian fascies (Mongolian face: slightly upward slanting eyes, creased eyelids, flattened face, flat nose).
• Generalized muscle atony.
• Mental handicap