Translocation and Down Syndrome: An Introduction
In trisomy 21 and
mosaic trisomy 21,
Down syndrome occurs because some or all of the cells have 47 chromosomes, including three chromosomes 21. However, approximately 3 percent to 4 percent of individuals with Down syndrome have cells containing 46 chromosomes, but still have the features associated with Down syndrome. How can this be? In such cases, material from one chromosome 21 gets stuck or translocated onto another chromosome, either prior to or at conception. In such situations, cells from individuals with Down syndrome have two normal chromosomes 21, but also have additional chromosome 21 material on the translocated chromosome. Thus, there is still too much material from chromosome 21, resulting in the features associated with Down syndrome. In such situations, the individual with Down syndrome is said to have translocation trisomy 21 or translocation Down syndrome.
Balanced Translocation and Down Syndrome
Most of the time, translocation Down syndrome occurs as the result of a random event. However, in some cases, translocation Down syndrome can be inherited. This occurs when one of the parents is a balanced carrier of the translocation.
A balanced translocation carrier of chromosome 21 is a person who has part of chromosome 21 attached (translocated) to another chromosome. However, this person does not have Down syndrome because he or she does not have any extra chromosome 21 -- it is just moved.
When reproduction occurs, if a parent passes on this translocated chromosome along with a normal chromosome 21, the child will develop Down syndrome. If a parent passes on this translocated chromosome along with the chromosome 21 that lost a section due to translocation, then the child will not develop Down syndrome, but will also be a balanced translocation carrier.
Genetic counseling can be sought to find the origin of the translocation.
It is important to realize that not all parents of individuals with translocation trisomy 21 are themselves balanced carriers. In such situations, there is no increased risk for Down syndrome in future pregnancies.
