chromosomal translocation chromosomal translocation in genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. this includes balanced and unbalanced translocation, with two main types: reciprocal, and robertsonian translocation. reciprocal translocation is a chromosome abnormality caused by exchange of parts between non-homologous chromosomes. two detached fragments of two different chromosomes are switched. robertsonian translocation occurs when two non-homologous chromosomes get attached, meaning that given two healthy pairs of chromosomes, one of each pair "sticks" and blends together homogeneously. chromosomal reciprocal translocation of the 4th and 20th chromosome. a gene fusion may be created when the translocation joins two otherwise-separated genes. it is detected on cytogenetics or a karyotype of affected cells. translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes). reciprocal translocations reciprocal translocations are usually an exchange of material between non-homologous chromosomes and occur in about 1 in 491 live births. such translocations are usually harmless, as they do not result in a gain or loss of genetic material, though they may be detected in prenatal diagnosis. however, carriers of balanced reciprocal translocations may create gametes with unbalanced chromosome translocations during meiotic chromosomal segregation. this can lead to infertility, miscarriages or children with abnormalities. genetic counseling and genetic testing are often offered to families that may carry a translocation. most balanced translocation carriers are healthy and do not have any symptoms. it is important to distinguish between chromosomal translocations that occur in germ cells, due to errors in meiosis (i.e. during gametogenesis), and those that occur in somatic cells, due to errors in mitosis. the former results in a chromosomal abnormality featured in all cells of the offspring, as in translocation carriers. somatic translocations, on the other hand, result in abnormalities featured only in the affected cell and its progenitors, as in chronic myelogenous leukemia with the philadelphia chromosome translocation. nonreciprocal translocation nonreciprocal translocation involves the one-way transfer of genes from one chromosome to another nonhomologous chromosome. robertsonian translocations robertsonian translocation is a type of translocation caused by breaks at or near the centromeres of two acrocentric chromosomes. the reciprocal exchange of parts gives rise to one large metacentric chromosome and one extremely small chromosome that may be lost from the organism with little effect because it contains few genes. the resulting karyotype in humans leaves only 45 chromosomes, since two chromosomes have fused together. this has no direct effect on the phenotype, since the only genes on the short arms of acrocentrics are common to all of them and are present in variable copy number (nucleolar organiser genes). robertsonian translocations have been seen involving all combinations of acrocentric chromosomes. the most common translocation in humans involves chromosomes 13 and 14 and is seen in about 0.97 / 1000 newborns. carriers of robertsonian translocations are not associated with any phenotypic abnormalities, but there is a risk of unbalanced gametes that lead to miscarriages or abnormal offspring. for example, carriers of robertsonian translocations involving chromosome 21 have a higher risk of having a child with down syndrome. this is known as a 'translocation downs'. this is due to a mis-segregation (nondisjunction) during gametogenesis. the mother has a higher (10%) risk of transmission than the father (1%). robertsonian translocations involving chromosome 14 also carry a slight risk of uniparental disomy 14 due to trisomy rescue. role in disease some human diseases caused by translocations are: cancer: several forms of cancer are caused by acquired translocations (as opposed to those present from conception); this has been described mainly in leukemia (acute myelogenous leukemia and chronic myelogenous leukemia). translocations have also been described in solid malignancies such as ewing's sarcoma. infertility: one of the would-be parents carries a balanced translocation, where the parent is asymptomatic but conceived fetuses are not viable. down syndrome is caused in a minority (5% or less) of cases by a robertsonian translocation of the chromosome 21 long arm onto the long arm of chromosome 14. chromosomal translocations between the sex chromosomes can also result in a number of genetic conditions, such as xx male syndrome: caused by a translocation of the sry gene from the y to the x chromosome by chromosome overview of some chromosomal translocations involved in different cancers, as well as implicated in some other conditions, e.g. schizophrenia, with chromosomes arranged in standard karyogram order. abbreviations: all – acute lymphoblastic leukemia aml – acute myeloid leukemia cml – chronic myelogenous leukemia dfsp – dermatofibrosarcoma protuberans human karyotype with annotated bands and sub-bands as used for the nomenclature of chromosomal abnormalities. it shows dark and white regions as seen on g banding. each row is vertically aligned at centromere level. it shows 22 homologous autosomal chromosome pairs as well as both the female (xx) and male (xy) versions of the two sex chromosomes. further information: karyotype denotation the international system for human cytogenetic nomenclature (iscn) is used to denote a translocation between chromosomes. the designation 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 refers to regions, bands and sub-bands seen when staining the chromosome with a staining dye.