Deletion

Deletions are unbalanced structural aberations (= they cause changes in the original amount of DNA) in which a part of the DNA is missing. They cause loss of genetic material.
==Types of deletions:==
: Terminal deletions: terminal deletions occur at the ends of a chromosome
: Intercalary deletions: intercalary deletions occur within the chromosome - from the interior of the chromosome
: microdeletions: cause loss of just a small amount of genetic material
==Causes of deletions:==
Deletions can be caused by chromosomal breaks (generally leading to the loss of the acentric chromosome fragment), by unequal crossing-over (generation of deletions, but also duplications), as a consequence of abnormal segregation of chromosomes with balanced translocations or inversions. During pairing of chromosomes during meiosis the unpaired region generated by a deletion results in formation of a deletion/compensation loop.
==Genetic consequences of deletions:==
*a heterozygote becomes monosomic for the part of genetic information, which is located on the corresponding fragment of normal chromosome
*haploinsuficiency: inability of the single copy of genetic material to maintain the functions, for which two copies of DNA are necessary
*LOH ("loss of heterozygosity"): loss of the dominant alele
*gene deletion and contiguous gene syndrome (= syndrome caused by abnormalities of 2 or more genes, that are located next to each other on the chromosome)
==Detection:==
Big deletions can be discovered by normal cytogenetic methods. Nowadays the most commonly used assay for deletion detection is the CGH (Comparative genome analysis).
==Diseases/Syndromes:==
Interstitial deletions:
*DiGeorge syndrome: microdeletion 22q11
*Prader-Willi/ Angelman syndrome: 15q11-q13
*Williams syndrome: 7q11
Terminal deletions:
*Cri-du chat: del5p15