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:Edit
- 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:Edit
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:Edit
- 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)
Big deletions can be discovered by normal cytogenetic methods. Nowadays the most commonly used assay for deletion detection is the CGH (Comparative genome analysis).
- DiGeorge syndrome: microdeletion 22q11
- Prader-Willi/ Angelman syndrome: 15q11-q13
- Williams syndrome: 7q11
- Cri-du chat: del5p15