chromosomal translocation

In genetics, a chromosome translocation is the interchange of parts between nonhomologous chromosomes. It is detected on cytogenetics or a karyotype of affected cells. There are two main types, reciprocal and Robertsonian. Also, translocations can be balanced (in an even exchange of material with no genetic information extra or missing) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).

Reciprocal Translocations

Usually resulting in an exchange of material between nonhomologous chromosomes, reciprocal translocations are found in about 1 in 600 newborns. Such translocations are usually harmless and may be found through prenatal diagnosis. However, carriers of balanced reciprocal translocations have increased risks create gametes with unbalanced chromosome translocations leading miscarriages or children with abnormalities. Genetic counseling and genetic testing is often offered to families that may carry a translocation.

Robertsonian translocations

This type of rearrangement involves two acrocentric chromosomes that fuse near the centromere region with loss of the short arms. The resulting karyotype has only 45 chromosomes since two chromosomes have fused together. Robertsonian translocations have been seen involving all combinations of acrocentric chromosomes. The most common translocation involves chromosomes 13 and 14 is seen in about 1 in 1300 persons, making it the most common chromosome rearrangment in humans. Like other translocations, carriers of Robertsonian translocations are phenyotypically normal, but there is a risk of unbalanced gametes and therefore miscarriages or abnormal offspring. For exmple, carriers of robertsonian translocations involving chromosome 21 have a higher chance to have a child with Down syndrome. Some diseases caused by translocations are:

Cancer - several forms of cancer are caused by translocations; this has been described mainly in leukemia (acute myelogenous leukemia and chronic myelogenous leukemia).
Infertility (one of the would-be parents carries a balanced translocation, where the parent is asymptomatic but conceived fetuses are not viable).
Down's syndrome is caused in a minority (5% or less) of cases by a Robertsonian translocation.

Notation

The shorthand t(A;B)(p1;q2) is used to denote a translocation between chromosome A and chromosome B. The information in the second set of parentheses, when given, gives the precise location within the chromosome for chromosomes A and B respectively—with p indicating the short arm of the chromosome, q indicating the long arm, and the numbers after p or q giving the distance in centimorgans from the centromere.