A Robertsonian translocation is a chromosomal abnormality that can lead to a variety of diseases in humans. When a piece of one chromosome breaks off and fuses with another, it’s called a chromosome break. This rearrangement must involve chromosomes 13, 14, 15, 21, and 22, which are acrocentric, meaning one chromosomal arm is much shorter than the other, in order to be considered a Robertsonian translocation. Although this translocation does not always cause immediate problems, it can result in too many copies of a particular chromosome in a person’s offspring. This chromosomal abnormality can lead to conditions like Down syndrome.
The genetic material of humans is stored in the nuclei of cells in the form of deoxyribonucleic acid (DNA). Normal humans have two sex chromosomes as well as two copies of 22 other chromosomes known as autosomal chromosomes, and this information is partitioned into different segments called chromosomes. At the centromere, a segment called the long arm is joined to a segment called the short arm on each chromosome. A piece of one of these chromosomes can sometimes break off and fuse with another. A translocation is the term for this phenomenon.
A Robertsonian translocation is one type of translocation. This translocation only occurs between chromosomes 13, 14, 15, 21, and 22, which differ from other chromosomes in that they have elongated long arms and truncated short arms that do not carry much important genetic information. The long arm of one chromosome joins the long arm of another chromosome in a Robertsonian translocation, and the short arms are effectively lost in the shuffle. This switch can happen between homologous chromosomes, such as the two different copies of chromosome 14, or between non-homologous chromosomes, such as chromosomes 14 and 22.
Robertsonian translocations aren’t always problematic right away. Because this genetic information was not critical, the loss of the short arms of the chromosomes causes no problems. Both of the original long arms can still be found in the nucleus, though they are connected in a different way. Robertsonian translocated cells can even go through mitosis, which is the process by which body cells divide to produce new cells.
When cells divide for the purpose of meiosis, a critical step in sexual reproduction, the problem with a Robertsonian translocation usually occurs. Meiotic cells contain only half of the genetic material of their parent cells; in humans, these cells form eggs and sperm. A Robertsonian translocation during meiosis could lead to an unequal distribution of genetic material.
Down syndrome, a disease that occurs when an individual has three copies of chromosome 21, is perhaps the best example of how a Robertsonian translocation can cause problems during meiosis. A Robertsonian translocation can cause Down syndrome if, for example, the long arm of chromosome 21 in the mother joins the other long arm of chromosome 21. During meiosis, one of the resulting eggs may receive the fused copy, giving it two copies of chromosome 21, while the other egg receives none. Normally, chromosome 21 would have been duplicated in each egg. 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 after fertilization by a normal sperm.