Down syndrome: causes and risk factors

Down syndrome is caused when a person’s genetic code has an extra copy of chromosome 21 (or part of it). Since chromosomes normally come in pairs, this is often called Trisomy 21.

It is not always clear why this anomaly occurs. In most cases, this is a random event when a sperm fertilizes an egg, although some risk factors for Down syndrome have been identified and there is a type of inherited disorder.

Causes

Trisomy for a specific chromosome, including chromosome 21, is the result of poor division of the sperm or egg before conception. Each of the three types of Down’s syndrome has a slight nuance in exactly how it is caused:

• Complete Trisomy 21: Chromosomes line up to divide and create eggs or sperm in a process called meiosis. With this type of Down syndrome, nondisjunction occurs. That is, an egg has two 21st chromosomes rather than one. Once fertilized, this egg then has a total of three chromosomes. This is the most common way of occurrence of Down syndrome.
• Translocation trisomy 21: In a translocation, there are two copies of chromosome 21, but additional material from a third chromosome 21 is attached (translocated to) another chromosome. This type of Down syndrome can occur before or after conception and is the form that can sometimes be passed on (hereditary).
• Mosaic trisomy 21: It is the less common form of Down syndrome. It occurs after conception for unknown reasons and differs from the other two types of Down’s syndrome in that only certain cells have an extra copy of chromosome 21. For this reason, the characteristics of a person with mosaic Down syndrome do not. are not as predictable as those with complete trisomy 21 and translocation. They may seem less obvious depending on which cells and how many cells have a third chromosome 21.

Genetic

Only one type of Down syndrome, translocation Down syndrome, is considered inherited. This type is very rare. Of these, only a third are believed to have inherited the translocation.

A translocation that will ultimately lead to a child with Down syndrome often takes place when that child’s parent is conceived. Part of a chromosome breaks and attaches to another chromosome during cell division. This process results in three copies of chromosome 21, with one copy attached to another chromosome, often chromosome 14.

This abnormality will not affect the normal development and function of the parent because all the genetic material required on the 21st chromosome is present. This is called a balanced translocation. However, when a person with a balanced translocation conceives a child, it is possible that that child will have an extra chromosome 21 and therefore be diagnosed with Down syndrome.

There is an increased likelihood that parents of a child with Down syndrome due to a translocation will have other children with the disease. It is also important that parents of a child who has had a translocation know that their other children may be carriers and are at risk of having a baby with Down syndrome in the future.

If a woman with Down’s syndrome becomes pregnant, she is at increased risk of having a baby with Down’s syndrome, but she is just as likely to have a child without the disease.

Risk factors

There are no environmental factors such as toxins or carcinogens that can cause Down syndrome, and lifestyle choices (like drinking, smoking, or taking drugs) don’t play a role. The only known non-genetic risk factor for having a child with Down syndrome is what is sometimes called advanced maternal age (over 35 years old).

This does not mean, however, that having a baby before age 35 is a reliable strategy for preventing Down syndrome. About 80% of children with Down’s syndrome are born to women under the age of 35.

Here’s how the risk of Down’s syndrome increases with the mother’s age: