Down's Syndrome - Causes

Chromosome #21 can be damaged, for example, if one parent has two chromosomes at location 21, rather than one. A child will then get two #21 chromosomes from one parent and one #21 chromosome from the other parent, for a total of three #21 chromosomes. This form of Down's syndrome is called trisomy 21, meaning that the #21 chromosome has three units rather than two. Overall, the child has forty-seven chromosomes, rather than forty-six chromosomes. This pattern of changes occurs in more than 90 percent of all Down's patients.

Chromosome #21 can also be damaged during cell replication. Cells grow and reproduce over time. As they do so, they make copies of the original chromosomes from the parents. Sometimes, the chromosomes from the parents are normal but a problem occurs when the new cells are reproducing. Two normal chromosomes at location 21 become three chromosomes in some cells.

This condition is known as a mosaic disorder. People with this disorder have some cells containing forty-six chromosomes, and some containing forty-seven chromosomes. They may have less severe symptoms than a person whose cells all contain forty-seven chromosomes. A mosaic condition occurs rarely. Less than 2 percent of all Down's cases are caused by mosaic disorder.

Chromosome #21 can also be damaged when a normal chromosome from the parent breaks into two pieces. One piece of the chromosome becomes attached to another chromosome. This process is called translocation. Translocation occurs in about 3 to 4 percent of all Down's patients.

Down's syndrome occurs in about 1 out of every 800 to 1000 births. It affects an equal number of females and males. The most common cause of Down's syndrome is an extra #21 chromosome in the mother's egg. As a woman grows older, she faces a greater risk of having a baby with Down's syndrome. For example, a woman in her twenties faces a risk of about 1 in 4,000. By the age of thirty-five, that risk has increased to 1 in 400 and, by the age of forty-five, to 1 in 35.