Down syndrome is a genetic condition resulting from an extra full or partial copy of chromosome 21. This additional genetic material alters a person’s development. Human cells normally contain 23 pairs of chromosomes that hold our genetic code. In Down syndrome, this blueprint includes extra material from the 21st chromosome, leading to the characteristics associated with the condition. It is not an illness or disease but a naturally occurring chromosomal arrangement that affects people of all races and economic levels.
The Three Genetic Types of Down Syndrome
There are three distinct genetic types of Down syndrome. The most common form, accounting for approximately 95% of all cases, is Trisomy 21. In this type, every cell in the body has three separate copies of chromosome 21 instead of the usual two. This form is also known as standard or free Trisomy 21.
A less common variation is Translocation Down syndrome, which makes up about 3-4% of cases. In this type, the total number of chromosomes in the cells remains 46. However, a full or partial extra copy of chromosome 21 is attached to another chromosome, such as chromosome 14.
The rarest form is Mosaic Down syndrome, identified in about 1-2% of individuals with the condition. Mosaicism is characterized by a mixture of two cell types. Some cells have the usual 46 chromosomes, while other cells have 47, containing the extra copy of chromosome 21. The percentage of cells with the extra chromosome can vary significantly among individuals and throughout different tissues in the body.
How Different Types of Down Syndrome Occur
The genetic variations of Down syndrome arise from events during cell division. Trisomy 21 and Mosaic Down syndrome are caused by an error called nondisjunction, where a chromosome pair fails to separate properly during the formation of reproductive cells. For Trisomy 21, this nondisjunction of chromosome 21 happens before or at conception. While advanced maternal age is a known factor that increases the chance of nondisjunction, anyone can have a child with Down syndrome.
In Mosaic Down syndrome, the nondisjunction event occurs in one of the initial cell divisions after fertilization. The timing of this error determines the proportion of affected cells in the body.
Translocation Down syndrome can occur spontaneously at conception or be passed down from a parent. About one-third of translocation cases are inherited from a parent who is a “balanced carrier.” A balanced carrier has the correct amount of genetic material, but a piece of their chromosome 21 is attached to another chromosome. Because they have no extra genetic material, they are unaffected but have an increased chance of having a child with Down syndrome.
Physical and Developmental Characteristics
Common physical characteristics of Down syndrome include:
- Low muscle tone (hypotonia) at birth
- A single deep crease across the center of the palm
- A slightly flattened facial profile
- Upward-slanting eyes
- A short neck
- Small hands and feet
Individuals with Down syndrome also face certain health considerations. About half are born with a heart defect, and there is an increased likelihood of hearing, vision, and thyroid issues. Developmentally, most individuals experience some degree of intellectual and developmental delay, though the range of abilities is wide. The specific type can sometimes influence these characteristics, as individuals with Mosaic Down syndrome may have fewer associated traits, but this is not always the case.
Diagnostic Processes
Down syndrome can be identified both before and after birth. During pregnancy, prenatal screening tests like non-invasive prenatal testing (NIPT) can assess the chance of the condition but are not definitive. For a conclusive diagnosis, tests like amniocentesis or chorionic villus sampling (CVS) analyze the baby’s chromosomes.
After birth, a diagnosis is often suspected based on physical traits. A laboratory test called a karyotype analysis confirms the diagnosis and determines the specific genetic type. This test involves taking a blood sample to create a visual map of the baby’s chromosomes.
Determining the specific type is particularly important in cases of Translocation Down syndrome. This information is valuable for parental genetic counseling, as it can indicate if the translocation was inherited and help assess the chances for future pregnancies.