Deletion on a chromosome

In genetics, a deletion (also called gene deletion, deficiency, or deletion mutation) is a mutation (a genetic aberration) in which a part of a chromosome or a sequence of DNA is missing. Deletion is the loss of genetic material. Any number of nucleotides can be deleted, from a single base to an entire piece of chromosome.^[1] Deletions can be caused by errors in chromosomal crossover during meiosis. This causes several serious genetic diseases.

[edit] Causes

Causes include the following:

• Losses from translocation
• Chromosomal crossovers within a chromosomal inversion
• Unequal crossing over
• Breaking without rejoining

For synapsis to occur between a chromosome with a large intercalary deficiency and a normal complete homolog, the unpaired region of the normal homolog must loop out of the linear structure into a deletion or compensation loop.

[edit] Types

Types of deletion include the following:

• Terminal Deletion - a deletion that occurs towards the end of a chromosome.
• Intercalary Deletion / Interstitial Deletion - a deletion that occurs from the interior of a chromosome.

[edit] Effects

Small deletions are less likely to be fatal; large deletions are usually fatal - there are always variations based on which genes are lost. Some medium-sized deletions lead to recognizable human disorders.

Deletion of a number of base pairs that is not evenly divisible by three will lead to a frameshift mutation, causing all of the codons occurring after the deletion to be read incorrectly during translation, producing a severely altered and potentially nonfunctional protein.

Deletions are responsible for an array of genetic disorders, including some cases of male infertility and two thirds of cases of Duchenne muscular dystrophy.^[1] Deletion of part of the short arm of chromosome 5 results in a syndrome called Cri du chat,^[1] French for "cry of the cat" syndrome. It is found in approximately 1 in 50,000 live births. The surviving infants have a distinctive cry, severe mental retardation, and shortened life span.

[edit] See also

• Chromosome abnormalities
• Null allele
• List of genetic disorders
• Medical genetics

[edit] References

1. ^ ^{a b c} Lewis R. 2005. Human Genetics: Concepts and Applications, 6th Ed. McGraw Hill, New York.