Down Syndrome Information: Causes of Down Syndrome

Down syndrome is the result of an extra copy of chromosome 21. There are 23 sets of chromosomes in the average, healthy, "normal" person, with two chromosomes present per set. Therefore, people with Down syndrome have three chromosomes 21, and forty-seven chromosomes in total, as opposed to the forty-six found in most people.

This extra, or "copied" chromosome in people with Down syndrome is caused by one of three chromosomal phenomenon. These abnormalities are called non-disjunction, mosaicism, and translocation. Depending on the type of Down syndrome a person has, the extra chromosome may be found in all of his or her cells, or only in some, however all people with Down syndrome will have some occurrence of the extra chromosome 21.

Down Syndrome Information
Causes: Non-Disjunction

The most common chromosomal anomaly that causes Down syndrome is called non-disjunction. This is the form of Down syndrome that is also known as Trisomy 21, a term that, because of its frequency, is often used synonymously with the condition, although two other form of Down syndrome exist. It is caused by a failure of the pair of chromosome 21 to separate at conception, either in the sperm or the egg. Therefore, when the unseparated pair meets their match in the corresponding chromosomes of the other parent, three copies of chromosome 21 are present in the embryo.

Non-disjunction is by far the most prevalent cause of Down syndrome. In fact, this malfunction of cell division is responsible for approximately 95% of all cases. Children born with Down syndrome due to non-disjunction possess the third copy of chromosome 21 in every cell in their bodies.

There is no definitive cause for non-disjunction, at least, not one that is currently understood. There does, however, appear to be a correlation between incidence of non-disjunction and the age of the mother. It can also be seen to occur in conjunction with the age of the father, however, these cases account for approximately 5% of the occurrences. This is why more diagnostic testing is encouraged in older mothers than younger mothers, and in the U.S., amniocentesis is becoming fairly standard in mothers over the age of thirty-five.

Down Syndrome Information
Causes: Mosaicism

Similar to non-disjunction, mosaicism is also a malfunction of the division of the pair of the chromosome 21. However, mosaicism takes place shortly after egg fertilization, where non-disjunction occurs at conception. This process is very rare, and accounts for less than 2% of all cases of Down syndrome.

Termed "mosaicism" because of a mosaic like appearance to the cells when viewing them under a microscope, people with Down syndrome due to this particular cell division error are thought to be less severely affected by the process. However, because Down syndrome in all its forms is so varying in severity from person to person, this is more of an impression than a fact. What is known, however, is that mosaicism occurs in the early stages of embryo development, in the first post-fertilization cell divisions, and not at conception. Because of this, there is a mixture to two different types of cells in people with Down syndrome due to mosaicism, some cells with forty-six chromosomes total, and some with forty seven.

Down Syndrome Information
Causes: Translocation

The third type of cell division problem responsible for the occurrence of Down syndrome is called translocation. Translocation is a different type of chromosomal malfunction from non-disjunction and mosaicism.

In translocation, there are initially 23 "normal" pairs of chromosomes present in the cells until, during cell division, a portion of one of the chromosome 21 breaks off and finds itself attached to another chromosome. There are still forty-six chromosomes seen in people with Down syndrome due to translocation, but that extra piece of the chromosome 21 present in the cell causes some of the features of Down syndrome in the child.

Like non-disjunction, translocation occurs either prior to or at conception. It is the cause of the condition in approximately 3% of people with Down syndrome. The age of the mother is not considered to be a factor in translocation, although there may be a genetic factor that is carried in one of the parents. Because of this, parents with a child with Down syndrome due to translocation run a greater risk of having a second child with the condition than those with a child born with it due to non-disjunction or mosaicism.

More Information about Down Syndrome
Down Syndrome Information: What is Down Syndrome?
Down Syndrome Information: Diagnosis of Down Syndrome
Down Syndrome Information: Characteristics of Down Syndrome
Down Syndrome Information: What Special Care do People with Down Syndrome Need?
Down Syndrome Information: Frequently Asked Questions
Down Syndrome Information: A Personal Account