Transfer (or translocation) is the process of moving genes from one location to another on a chromosome. Translocation can be between two non-homologous chromosomes (crossing over), or it can be between two homologous chromosomes (gene conversion). In eukaryotes, translocation is a type of mutation.
Mutations are changes in the DNA sequence of a gene. Most mutations are harmful, but some can have beneficial effects. A change in the DNA sequence can alter the function of a gene. A translocation is a type of mutation in which a piece of DNA is moved from one location to another on the same chromosome or to a different chromosome.
Translocations can be caused by errors during replication or by exposure to certain chemicals or radiation. Some translocations are inherited, while others occur spontaneously. Inherited translocations are responsible for several human diseases, including cancer. Spontaneous translocations often lead to cancer in animals and plants.
A chromosomal aberration is an abnormality in the number or structure of chromosomes. Aberrations can be caused by exposure to certain chemicals or radiation, as well as by errors during cell division. Chromosomal aberrations that involve the loss or gain of whole chromosomes are called aneuploidy; those that involve deletions, duplications, or other rearrangements of chromosome segments are called structural aberrations; and those that involve changes in chromatin structure but not in DNA sequence are called epigenetic aberrations.
Aneuploidy occurs when there is an abnormal number of chromosomes due to either extra copies (trisomy) or missing copies (monosomy) of particular chromosomes; it may also result from nondisjunction, which is when pairs of homologous chromosomes fail to separate during meiosis I so that both members go into the same daughter cell instead of one going into each daughter cell as normal.* Structural aberrations include deletions (a missing segment), duplications (an extra copy), insertions (an interstitial segment), reciprocal translocations (exchange between nonsister chromatids), Robertsonian translocations*(fusion between two acrocentric*chromosomes), and ring formations.* Epigenetic changes refer broadly to any heritable modification made without altering the nucleotide sequence—for example, methylation patterns that affect gene expression but not necessarily DNA sequence.* Many epigenetic modifications accumulate with age and contribute to aging-related diseases such as cancer.*
The most common type of cancer-causing chromosomal aberration involves gains or losses within individual chromosomes rather than whole-chromosome gains or losses; these types are termed numerical aberrations.* The best characterized numerical aberration affecting tumorigenesis*is trisomy 21, otherwise known as Down syndrome.* This condition arises when nondisjunction results in an embryo having three instead of two copiesof chromosome 21; people with Down syndrome thus have 47 total chromosomes insteadof 46.* Trisomy 21 predisposes individualsto developing leukemia and Alzheimer disease*, among other conditions.* Deletionof portionsof chromosome 5q is associated with myelodysplastic syndromes*, while deletionof partsof chromosome 7 leads tonon–small cell lung cancer*. Balancedtranslocationsinvolving breakpointswithin oncogenes*or tumor suppressorgenes*have also been implicatedin human cancers: for example,the Philadelphiachromosome*was first identifiedin patientswith chronic myelogenousleukemiaand results from atranslocationbetweenchromosomes 9 and 22.* This particularaberrationfuses ablood vessel growth factorgene locatedon chromosome9 with abloom syndromehelper helicasegene foundon chromosome22—the resultinghybrid proteinhas uncontrolledcell proliferationactivityand causes leukemiaby producingtoo many white blood cells*. Althoughmost spontaneoustranslocationsare not directly involvedin tumor formation*,they do increasethe risk for developingcancer laterin life by increasingthe rate at whichother mutationsaccumulate*.