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What is Aneuploidy?

Meaning and Definition of Aneuploidy

Aneuploidy  is the second largest category of chromosomal mutations iAneuploidynvolving changes in the number of chromosomes. The addition or loss of one or more chromosomes upsets the balance in cells and, in most cases, is not compatible with life.

Causes of Aneuploidy

Aneuploidies are primarily caused by nondisjunction during meiosis or mitosis . Nondisjunction is an error in the process of segregation , where two chromosomes or chromatids go to one pole and neither go to the opposite pole in the meiotic or mitotic division.

The aneuploid classification is linked to the number of copies of a specific chromosome that is present in an aneuploid state. Monosomal aneuploidy , identified by the 2n–1 condition, is one in which only one copy of the specific chromosome is present, rather than the usual two copies found in diploids.

Normally, monosomatics are incompatible with life, but some manage to survive, as is the case of patients with Turner syndrome , represented by X0. These people have 44 autosomal chromosomes and in the pair of sex chromosomes they have only one X chromosome.

Those affected by Turner syndrome are phenotypically characterized as women, infertile, short stature, flat neck, among other characteristics.

The trisomic aneuploid, identified by the 2n + 1 condition, is one with an extra copy of a specific chromosome. In diploids, the trisomic condition can lead to chromosomal abnormalities or death, but some trisomies are compatible with life.

Klinefelter syndrome , represented by XXY, is a trisomic condition of the sex chromosomes whose sufferers are male, infertile, thin, tall, with a lower than average IQ, may develop breasts and have small testicles. Even in human sex chromosome trisomies , there are syndromes represented by XYY and XXX, where individuals have normal phenotypes and are fertile, not transmitting the condition to the next generation.

Chromosomal Trisomies

In autosomal chromosomal trisomies , the most common type is chromosomal trisomy 21, or Down syndrome. It affects one in 800 natives, and the phenotype of those affected includes mental retardation, short stature, small square hands and feet, muscular hypotonia, cardiac abnormality, small low-set ears, eyes with oblique slits in the lids, large tongue , protruding and striated.

Affected women are usually fertile and can have normal children or children with the syndrome . Most men are sterile. The incidence of Down syndrome is related to maternal age; Older women are more likely to have children with Down syndrome.

Two other human autosomal trisomies that can survive are chromosome 18, known as Edwards syndrome, and chromosome 13, known as Patau syndrome. Both have serious physical and mental abnormalities, and affected children die within weeks or months.

Nulisomal aneuploidy , represented by the 2n–2 condition, indicates the loss of a pair of chromosomes and in diploids is considered lethal. There is also uniparental disomal aneuploidy, which has a normal number of chromosomes (2n), but the two chromosomes in the pair come from only one parent.

Cytogenetics

The area of ​​science that studies aneuploidy is cytogenetics. Cytogenetics is the field of genetics that studies chromosomes, their structure, composition and role in the evolution and development of diseases. One of the most used methods for the identification of chromosomes is the karyotype.

The test is performed from a cell culture that allows dividing cells to be obtained and then chromosomes to be stained with bands. Through chromosomal bands it is possible to identify numerical and/or structural chromosomal aberrations, balanced or unbalanced, total or partial.

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