A chromosome abnormality reflects an abnormality of chromosome number or structure. Chromosome abnormalities usually occur when there is an error in cell division following meiosis or mitosis. There are many types of chromosome abnormalities. However, they can be organized into two basic groups, numerical and structural abnormalities. This article or section does not cite its references or sources. ... In biology, meiosis is the process that allows one diploid cell to divide in a special way to generate haploid cells in eukaryotes. ... Mitosis divides genetic information during cell division. ...

Numerical abnormalities

Main article: Aneuploidy

When an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy). An example of a condition caused by numerical abnormalities is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Turner Syndrome is an example of monosomy 13 the individual is born with only one sex chromosome, an X. Aneuploidy is a chromosomal state in which abnormal numbers of specific chromosomes or chromosome sets exist within the nucleus. ...

Structural abnormalities

When the chromosome's structure is altered. This can take several forms:

• Deletions: A portion of the chromosome is missing or deleted. Known disorders include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder.
• Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Known disorders include Bloom and Rett syndrome.
• Translocations: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere; these only occur with chromosomes 3, 14, 15, 21 and 22.
• Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.
• Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.

Chromosome instability syndromes are a group of disorders characterized by chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies. A genetic deletion is a genetic aberration in which part of a chromosome is missing. ... 4p- karyotype Wolf-Hirschhorn syndrome, also known as deletion 4p and 4p- syndrome was first described in 1961 by U. Wolf and K. Hirschhorn and their co-workers. ... Summary Jacobsen Syndrome, also known as 11q deletion, is a congenital disorder that occurs due to a partial deletion of the terminal band on chromosome 11. ... A duplication Chromosomal duplication occurs when a section of DNA is accidentally duplicated when a chromosome is copied. ... Bloom syndrome is a genetic condition characterized by prenatal growth delay and a butterfly rash in the mid-face region. ... Added to User:reverie98/Desk 71. ... Chromosomal translocation of the 4th and 20th chromosome. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. ... Formation of a ring chromosome. ...

Inheritance

Most chromosome abnormalities occur as an accident in the egg or sperm, and are therfore not inherited. Therefore, the abnormality is present in every cell of the body. Some abnormalities, however, can happen after conception, resulting in mosaicism (where some cells have the abnormality and some do not). Chromosome abnormalities can be inherited from a parent or be "de novo". This is why chromosome studies are often performed on parents when a child is found to have an abnormality. Biological inheritance is the process by which an offspring cell or organism acquires or becomes predisposed to characteristics of its parent cell or organism. ... Mosaicism In medicine (genetics), a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one patient, where usually one of the two is affected by a genetic disorder. ...

Reference

• NHGRI. 2006. Chromosome Abnormalities