Trisomies
A trisomy occurs when a chromosome that should normally appear in pairs is found in three copies.
For example, when a baby has three copies of chromosome 21 instead of the usual two, this condition is known as Trisomy 21, or Down syndrome.
Other common types of trisomy include Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome), in which each cell of the body carries three copies of chromosome 18 or 13, respectively.
Down Syndrome
Down syndrome is a chromosomal disorder characterized by intellectual disability, neurological impairments, distinctive facial features, heart defects, and vision and hearing problems.
The severity of these conditions varies among individuals.
Down syndrome is the most common genetic birth anomaly, occurring in approximately 1 in 800–1,000 births.
Average life expectancy is around 55 years, though it may vary depending on individual health factors.
Causes
In normal reproduction, both the egg (from the mother) and the sperm (from the father) contain 46 chromosomes.
During cell division, this number is halved, leaving each gamete with 23 chromosomes.
When fertilization occurs, the embryo receives a total of 46 chromosomes—half from each parent.
If an error occurs during this division, the embryo may inherit three copies of chromosome 21, leading to Trisomy 21 (Down syndrome).
Parents who have had one child with Down syndrome have an estimated 1.7% recurrence risk in subsequent pregnancies.
This does not significantly increase the risk of other birth anomalies or affect the likelihood of relatives having a child with Down syndrome.
In some cases, the extra chromosome 21 is attached to another chromosome; this is called translocation Down syndrome, accounting for 3–4% of cases. This form may be hereditary.
Certain parents carry a balanced Robertsonian translocation, where chromosome 21 is attached to another chromosome without affecting the parent’s own health.
Another type, mosaic Down syndrome (1–2% of cases), occurs when the chromosomal error happens after fertilization, resulting in a mix of normal and trisomic cells.
Common Physical Features of Down Syndrome
Children with Down syndrome may show some or all of the following characteristics:
- Upward-slanting eyes
- Small, curved ears
- Small mouth with relatively large tongue
- Flat nasal bridge and small nose
- Flattened neck and nape area
- Short neck
- Small hands
About 40–50% of children with Down syndrome have congenital heart defects, some requiring medication or surgery.
All infants should undergo pediatric cardiology evaluation and echocardiography within the first two months after birth.
Additionally:
- ~10% have intestinal anomalies requiring surgery.
- >50% experience hearing and vision problems, including strabismus, cataracts, nearsightedness or farsightedness.
- Thyroid disorders and leukemia risk are higher.
- Upper respiratory infections are more common.
- Intellectual disability is typically moderate (IQ 60–75), with <10% showing severe impairment.
Diagnosis and Management
Currently, there is no preventive or curative treatment for the chromosomal abnormality that causes Down syndrome.
After birth, diagnosis may be suspected based on physical findings, but it must be confirmed by a chromosomal analysis (karyotype), which reveals the extra copy of chromosome 21.
Prenatal Diagnosis
Down syndrome can be detected before birth through the following methods:
- Amniocentesis: Examination of fetal cells from amniotic fluid
- Chorionic Villus Sampling (CVS): Examination of placental tissue
- Fetal Ultrasound: Evaluation of characteristic anatomical findings
Chromosomal analysis of cells obtained via amniocentesis or CVS provides a 99.9% accurate diagnosis.
Down Syndrome Risk by Maternal Age
| Maternal Age | Down Syndrome Risk |
| 15–24 years | 1 in 1,300 |
| 25–29 years | 1 in 1,100 |
| 35 years | 1 in 350 |
| 40 years | 1 in 100 |
| 45 years and older | 1 in 25 |
In summary, Down syndrome is a genetic condition caused by an extra copy of chromosome 21.
While it cannot be prevented, early screening, accurate diagnosis, and multidisciplinary management help ensure the best possible health and developmental outcomes for affected children and their families.
