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Inherited Disorders and Birth Defects

Down syndrome

08/31/2007

Question:

I am the only one having a child with down`s syndrome in my whole family that includes relatives close and far and also no one in the time of my grandparents there was no one with down`s syndrome. Will my children who are normal have any possibility of having a kid with down`s syndrome?

Answer:

Down syndrome is caused by having an extra 21 chromosome. It is this extra chromosome that causes the problems that people with Down syndrome have, such as delayed development, mental retardation and having particular facial features. It was named after the doctor that first described it, Dr. Down, in 1866.

Every cell in the body has 46 chromosomes, except eggs and sperm that have 23. Chromosomes are the structures that carry genes that tell our body how to form and how to function. When cells divide to make new cells, half the chromosomes go to one cell and the other half go to the other cell. When eggs and sperm are made they should only have 23 chromosomes each, so that when they unite at conception, the embryo gets half their chromosomes from their mother and half from their father and has the correct number of chromosomes again - 46.

Sometimes, the cells do not divide correctly, and a new cell can get too many chromosomes or not enough chromosomes. This is called non-disjunction. This is what happens in Down syndrome, either the egg or sperm that made the developing embryo did not divide correctly. In the case of Down syndrome, the extra chromosome was a number 21.

Down syndrome is usually not inherited. This appears to be the case in your family as no other family members have had a child with Down syndrome. Researchers still do not know why non-disjunction occurs. We do know that non-disjunction occurs more frequently in mothers who are older. It may be that as women get older, such as over 35 or 40 years of age, the process that helps the cell divide correctly, becomes less efficient or less good at dividing correctly.

Because Down syndrome is usually not inherited, your other children should not have an increased chance of having a child with Down syndrome. However, there is a screening test, a blood test that can tell if a pregnancy is at higher risk to have a baby with Down syndrome. And, there are diagnostic tests, such as amniocentesis, that tells specifically about the unborn baby's chromosomes. Because your children have a sibling with Down syndrome, I would recommend they talk to a genetic counselor when they are thinking of having children.

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Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University