Aneuploidy
| Aneuploidy | |
|---|---|
| Specialty | Medical genetics  |
Aneuploidy is a condition in which the number of chromosomes is abnormal due to extra or missing chromosomes, in other words, it is a chromosomal state where the number of chromosomes is not a multiple of the haploid set. Normal diploid species have 2n chromosomes, where n is the number in the haploid set. Aneuploid individuals would have 2n-1 chromosomes (monosomy), 2n+1 chromosomes (trisomy), or some other such arrangement.
A change in the number of chromosomes can lead to a chromosomal disorder. Aneuploidy is common in cancerous cells.
The most commonly observed forms of aneuploidy
Monosomy
Monosomy is the presence of only one chromosome from a pair in a cell's nucleus. Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has two copies.
Human genetic disorders arising from monosomy are:
- X0-Only one X chromosome instead of the usual two (XX) seen in a normal female, also known as Turner syndrome
- cri du chat syndrome -- a partial monosomy caused by a deletion of the end of the short p (from the word petit, French for small) arm of chromosome 5
- 1p36 Deletion Syndrome -- a partial monosomy caused by a deletion at the end of the short p arm of chromosome 1
Disomy
A disomy is the presence of a pair of chromosomes. For diploid organisms, such as humans, it is the normal condition. For organisms that are normally triploid or above, disomy is an aneuploidy. It can also refer to cells that are normally haploid, such as gametes.
In uniparental disomy, the disomy refers to two copies of the chromosome from one of the parents (with no contribution from the other parent).
Trisomy
A trisomy is the presence of three, instead of the normal two, chromosomes of a particular numbered type in an organism. Thus the presence of an extra chromosome 21 is called trisomy 21. Most trisomies, like most other abnormalities in chromosome number, result in distinctive birth defects. Many trisomies result in miscarriage or death at an early age.
A partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes, or if one of the chromosomes has two copies of part of its chromosome. A mosaic trisomy is a condition where extra chromosomal material exists in only some of the organism's cells.
While a trisomy can occur with any chromosome, few embryos survive to birth with most trisomies. The most common types that survive to birth in humans are:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Trisomy 9
- Trisomy 8 (Warkany syndrome 2)
Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester.
Trisomy involving sex chromosomes in humans includes:
- XXX (Triple X syndrome)
- XXY (Klinefelter's syndrome)
- XYY (XYY syndrome)
Sources
This article incorporates public domain text from The U.S. National Library of Medicine.