What Causes Down Syndrome?

Down syndrome is a common and recognizable chromosomal condition that is diagnosed in the United States. Individuals with Down syndrome have cognitive and physical symptoms, causing mild to moderate intellectual disability and other deficits. Despite some physical and behavioral similarities, all individuals with Down syndrome have differing abilities. Down syndrome is caused by an extra chromosome, but the Special Olympics Arizona team is breaking down exactly what that means and some risk factors associated with it.

What Causes Down Syndrome?

Down syndrome is caused by an extra chromosome, either a partial or full extra chromosome, on chromosome 21. The extra chromosome will affect how a baby’s body is formed and develops during pregnancy and throughout their life. There are three different types of Down syndrome, categorized by the type of chromosomal mutation that is present in an individual.

Trisomy 21

The most common cause and type of Down syndrome is trisomy 21, in which an individual has three copies of chromosome 21 instead of the standard two copies in every cell. This occurs in about 95% of individuals with Down syndrome and is caused by abnormal cell division during the development of the sperm cell or the egg cell.

Translocation Down Syndrome

Down syndrome may also occur when a portion of chromosome 21 becomes attached, or translocated, to another chromosome. This can occur before or at conception. Individuals with translocation down syndrome have the usual two copies of chromosome 21, but they will also have genetic material from chromosome 21 attached to another chromosome in their cells. This type of Down syndrome affects less than 4% of all cases.

Mosaic Down Syndrome

The most rare form of Down syndrome is mosaic. Mosaicism occurs when only some cells contain the usual 46 chromosomes and some cells contain 47. The extra chromosomes in the cells with 47 chromosomes are located on chromosome 21, similar to trisomy 21, but they do not occur in every cell. This occurs in less than 1% of all cases and is caused by abnormal cell division after fertilization.

Down Syndrome Risk Factors

Some parents have a greater risk of having a child with Down syndrome, depending on risk factors. Some risk factors that increase the risk of having a child with Down syndrome include:

Maternal age: A woman’s chance of having a child with Down syndrome increases with age because older eggs have a greater risk of improper cell division. A woman’s risk of having a child with Down syndrome increases after 35 years of age, however statistics show that most children with Down syndrome are born to younger moms, simply because women under the age of 35 have more children.
Being carriers of the translocation gene for Down syndrome: Both men and women who are carriers of the translocation gene for Down syndrome are at risk of passing this gene onto their children.
Having one child with Down syndrome: Parents who already have one child with Down syndrome or who have Down syndrome themselves are at a greater risk for having another child with Down syndrome. A genetic counselor can provide parents with a risk analysis for having another child with Down syndrome.

Is Down Syndrome Inherited?

Most of the time, Down syndrome is not inherited; instead, it is mostly caused by a mistake in cell division during early development of the fetus. Translocation Down syndrome is the type of Down syndrome that can be passed from parent to child, whether the parent carries the translocation gene or has Translocation Down syndrome themselves. Translocation Down syndrome accounts for less than 4% of all cases, and only some of these cases were inherited from parents.

Can Down Syndrome Be Prevented?

Down syndrome cannot be prevented. If you are at high risk for having a child with Down syndrome or you already have one child with Down syndrome, consulting a genetic counselor before getting pregnant can help you determine your chances of having another child with Down syndrome. A genetic counselor can help you understand your chances and can also provide information on the prenatal tests that are available and explain the pros and cons of testing.

What Causes Down Syndrome?