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Extra or Missing Chromosomes

In Down syndrome besides regular trisomy there is a chance of having a translocation of chromosome no.21 with chromosomes 13, 14,or 15. One of the parents can be a balanced carrier of this translocation. I read that if a father is carrier then there is an increased chance of having an affected child than if the mother is a carrier of translocation. Why is this difference?

May 25, 2004

Good question. I knew about translocation mutations and Down syndrome but hadn't looked into it in too much detail before.

People usually have two copies of each chromosome (except for the sex chromosomes, X and Y). About 95% of Down syndrome cases are caused by having three copies of chromosome 21, as shown in the image to the right.

About 3-4% of cases are because some or all of chromosome 21 has been translocated or moved to another chromosome. For Down syndrome cases, the most common chromosome for part or all of 21 to be stuck to is 14, although 13, 15, 21, and 22 also happen.

A balanced carrier is someone who has the right amount of chromosome 21 but one of the pairs is stuck to another chromosome (see the image to the right). These people still have 2 copies of chromosome 21 and so don't have any Down syndrome symptoms. However, they can give their kids Down syndrome.

How can this happen? Let's use the carrier from Figure 2 as an example. Through normal sperm production, it is possible for his body to make a sperm with the chromosome made from chromosomes 14 and 21 and the "free" chromosome 21. When combined with a "normal" egg, a Down syndrome baby will be the result because the baby will have 3 copies of chromosome 21.

So, finally to your question. Theoretically, there should be a 33% chance for a normal, a 33% chance for a balanced and a 33% chance for a Down syndrome child. (Part of the reason for these numbers is that 3 other possibilities, 1 copy of either chromosome 14 or 21, or 3 copies of chromosome 14, result in a nonviable fetus that is aborted naturally.) Instead of the theoretical 1 in 3 chance of having a child with Down syndrome, the actual numbers are 12% if the carrier is the mother and 3% chance if the father is the carrier.

Neither chance is the theoretical 1 in 3 because a high percentage of fetuses with three copies of chromosome 21 are naturally aborted. Why the difference between men and women? Researchers don't seem to know although there is some talk about it being due to the fact that women are born with all of their eggs while men make new sperm from puberty on. Because of this, there seems to be a selection against chromosomal abnormalities in sperm production which means men would produce fewer sperm with the wrong amount of DNA.

By Dr. Barry Starr, Stanford University


Around 95% of Down syndrome cases look like this.


When Down syndrome looks like this, it could have been passed down from parent to child.