Deletion (Chromosome)

A genetic deletion is a genetic aberration in which part of a chromosome is missing. This causes several serious genetic diseases.

One example of genetic deletions is Cri du chat, also known as "cry of the cat" syndrome. It is found in approx. 1 in 50,000 live births. The surviving infants have a distinctive cry, severe mental retardation, and shortened life span.

Deletion is the loss of genetic material. Causes:
 * losses from translocation
 * crossovers within an inversion
 * unequal crossing over
 * Breaking without rejoining

Small deletions are less likely to be fatal; large deletions are usually fatal - but always, there is variation based on what genes are lost. Some medium-sized deletions lead to recognizable human disorders.

Types of Deletion:
 * Terminal Deletion - a deletion that occurs towards the end of a chromosome.
 * Intercalary Deletion - a deletion that occurs from the interior of a chromosome.

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.

A gene deletion, or deletion mutation is a mutation in which a part of a chromosome or a sequence of DNA is missing. Any number of nucleotides can be deleted, from a single base to an entire piece of chromosome (Lewis, 2005, p.226). Deletions can be caused by errors in chromosomal crossover during meiosis.

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 (Lewis, 2005, p.226). A deletion of part of the short arm of chromosome 5 results in Cri du chat (Lewis, 2005, p.256).

Reference

 * Lewis R. 2005.  Human Genetics: Concepts and Applications, 6th Ed.  McGraw Hill, New York.