A Robertsonian translocation is type of chromosomal abnormality that can cause a number of different human diseases. It occurs when a piece of one chromosome breaks off and fuses with another chromosome. In order to be considered a Robertsonian translocation, this rearrangement must involve chromosomes 13, 14, 15, 21, and 22, which are acrocentric, meaning that one chromosomal arm is much shorter than the other. Although this translocation does not always cause immediate problems, it can result in a person's offspring having too many copies of a given chromosome. Conditions such as Down syndrome can be caused by this chromosomal abnormality.
A human's genetic material is stored in the form of deoxyribonucleic acid (DNA) and is kept within the nuclei of cells. This information is partitioned into different segments called chromosomes, and normal humans have two sex chromosomes as well as two copies of 22 other chromosomes referred to as the autosomal chromosomes. Each chromosome is composed of a segment called a long arm that is joined to a segment called the short arm at the centromere. Sometimes a piece of one of these chromosomes can break off and fuse with another chromosome. This phenomenon is referred to as a translocation.
One type of translocation is called a Robertsonian translocation. This translocation only occurs between the so-called acrocentric chromosomes, including chromosomes 13, 14, 15, 21, and 22, which are different from the other chromosomes because they have elongated long arms and truncated short arms that do not carry much important genetic information. When a Robertsonian translocation occurs, the long arm of one chromosome joins to the long arm of another chromosome, and the short arms are effectively lost in the shuffle. This switch can occur between two homologous chromosomes, for example between the two different copies of chromosome 14, or between non-homologous chromosomes, for instance between chromosomes 14 and 22.
Robertsonian translocations do not always cause immediate problems. The loss of the short arms of the chromosomes does not cause any problems, because this genetic information was not critical. Both of the original long arms are still present in the nucleus, although they are joined in a different configuration. Cells with a Robertsonian translocation can even undergo mitosis, which is the process by which body cells divide to create new cells.
The problem with a Robertsonian translocation typically occurs after when cells divide for the purpose of meiosis, which is a critical step in sexual reproduction. Cells produced from meiosis have only half of the genetic material of the parent cell; in the human, these cells form into the eggs and sperm. During meiosis, a Robertsonian translocation could result in an unequal distribution of genetic material.
Perhaps the best example to illustrate how a Robertsonian translocation can cause problems during meiosis is seen in Down syndrome, a disease that occurs when an individual has three copies of chromosome 21. Down syndrome can be caused by a Robertsonian translocation if, for example, in the mother the long arm of chromosome 21 joined to the other long arm of chromosome 21. During meiosis one of the resulting eggs could get the fused copy, thus effectively having two copies of chromosome 21, while the other egg received no copies. Normally each egg would have received one copy of chromosome 21. After fertilization by a normal sperm, the resulting embryo could have the fused chromosome from the mother and one normal chromosome from the father, resulting in three effective copies of chromosome 21.