What is translocation Down syndrome?
Translocation Down syndrome refers to the type of Down syndrome that is caused by rearranged chromosome material. In this case, there are three # 21 chromosomes, just like there are in trisomy 21, but one of the 21 chromosomes is attached to another chromosome, instead of being separate. The extra copy of the # 21 chromosome is what causes the problems that are associated with Down syndrome. In translocation Down syndrome, the extra 21 chromosome may be attached to the #14 chromosome, or to other chromosome numbers like 13, 15, or 22. In some cases, two #21 chromosomes can be attached to each other.
Three percent to 4 percent of babies born with Down syndrome have translocation Down syndrome. If you had a roomful of 100 babies with Down syndrome, they would all look alike and have similar features and health problems. You would not be able to pick out the three or four that have a translocation.
Whenever a translocation is found in a child, the parents' chromosomes are studied to determine whether the translocation was inherited or not. If one parent has the translocation chromosome, then the physician knows the baby inherited the translocation from that parent. That parent will actually have 45 total chromosomes in each cell of their body, but they will be normal and healthy because they still have only two copies of each chromosome. When a person has a rearrangement of chromosome material, with no extra or missing chromosome material, they are said to have a "balanced translocation" or be a "balanced translocation carrier."
Parents with balanced translocations may have fertility problems (trouble becoming pregnant), miscarriages, or have an increased chance of having a child with health problems. Although the parent can donate the proper amount of genetic material (23 chromosomes) to a pregnancy, they also have a risk of donating too much or too little genetic material to a pregnancy. This is not something the parent can control or predict. The chance depends on the type of chromosome rearrangement and which chromosomes are involved. For example, if the translocation is between #14 and 21, there is a 10 percent to 15 percent chance for Down syndrome in a pregnancy if the mother is the translocation carrier, and a 3 to 5 percent chance if the father is the translocation carrier. The chances are different for men and women because sperm and eggs are produced differently. A woman is born with all of the eggs she will ever have, while men are constantly producing new sperm.
There is another important factor to remember when a parent is found to have a translocation. The parents' relatives (brothers, sisters) may also have inherited the translocation and, therefore, may have the same risks for problems with a pregnancy. For these reasons, it is recommended that people with chromosome rearrangements share this information with their relatives so that they can have the option of having their chromosomes studied.