Inherited Disorders and Birth Defects

Downs syndrome and Alzheimers

01/15/2004

Question:

My wife has just turned 40 and i am 45 years old. We just had our first child. We did not undergo any prenatal tests for any disorders. The new neighbors next door have a down`s syndrome infant. They are about our age. My wifes father was recently diagnosed with having Alzheimers disease. What is the chance that our doughter has Downs syndrome? is there a connection between my father-in-law`s illnesss and Down`s syndrome? How can we find out for sure?

Answer:

If your baby is healthy and has no unusual features or problems in growth and development, it would be extremely unlikely that your daughter has Down syndrome. Doctors are very familiar with this diagnosis, so I would assume if there was a problem or suspected problem, your doctor would have suggested that your baby have a chromosome analysis done.
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 being retarded and having particular facial features.
As you may know, every cell in the body has 46 chromosomes, except eggs and sperm that have 23. 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 what happens in Down syndrome. Either the egg or sperm that made the developing embryo did not divide correctly and the extra chromosome was a number 21. This is called non-disjunction.
Down syndrome is usually not inherited. 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 - because women are born with all the eggs they will ever produce stopped in the middle of their cell division before they are born - over 35 or 40 years, the process that helps the cell divide correctly, becomes less efficient or less good at dividing correctly. The chance of a woman having a baby with Down syndrome is about 1 in 400 at the age of 35 and is about 1 in 110 at age 40. There does not appear to be an association between the father’s age and the chance to have a baby with Down syndrome.
Currently, researchers think that the extra "gene dosage" from the third chromosome 21 of Down syndrome is the cause of Alzheimer`s disease in people with Down syndrome. This is NOT the cause of Alzheimer`s disease in people who do not have Down syndrome. There would be no reason to believe that your daughter would be at any more increased risk for Alzheimer’s disease than you or her mother. If you are really concerned about your daughter’s health, I would recommend that you talk to your pediatrician about your concerns.

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Response by:

Anne Matthews, RN, PhD
Director, Genetic Counseling and Family Studies
Associate Professor
Department of Genetics
School of Medicine
Case Western Reserve University