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Short Notes on Aneuploidy

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Aneuploidy Meaning

Aneuploidy is a type of chromosomal aberration, where there is one extra chromosome or one missing chromosome. Many cancerous cells have shown to be aneuploid.

Types of Aneuploidy

Hypoploidy- When one or more chromosomes are less in number compared to the normal individual (2n), e.g. 2n-1, 2n-2, etc.

  • Monosomy- When there is one chromosome less in the cells, e.g. Turner’s syndrome (XO)
  • Nullisomy- When there is loss of a pair of chromosomes, i.e. 2n-2. Often they do not survive.

Hyperploidy- Here there is an addition of one or more chromosomes.

  • Trisomy- When there is an extra chromosome in the cells, e.g. Klinefelter’s syndrome (XXY), Down’s syndrome trisomy of the 21st chromosome.
  • Tetrasomy- There is an addition of a pair of chromosomes, i.e. 2n+2.
  • Disomy- When there is one chromosome more in a monoploid organism, i.e. n+1.

Mosaic Variegated aneuploidy- The phenomena is also known as mosaicism. It occurs when some of the cells contain an abnormal number of chromosomes.

Aneuploidy Causes

Aneuploidy is caused due to non-disjunction of chromatids during cell division. When chromatids fail to segregate during mitotic or meiotic cell division, it leads to one cell with an extra chromosome and another with one less chromosome. It causes aneuploidy.

Nondisjunction occurs during anaphase of cell division when sister chromatids separate.

Mosaicism is caused when nondisjunction happens after fertilization during cell division in the zygote.

Aneuploidy and Polyploidy

Aneuploidy differs from polyploidy. In polyploidy, there is an increase in one or more complete sets of chromosomes. Polyploidy occurs when cytokinesis does not follow karyokinesis. Polyploidy is common in plants. E.g. Triploid, Hexaploid, etc.

Euploidy is also referred to as variation in the complete sets of chromosomes, i.e. multiple of haploid sets of chromosomes. Euploidy above the number of diploid cells is termed as polyploidy.

Aneuploidy Disorders

Aneuploidy in humans leads to various chromosomal disorders in humans and also leads to miscarriages. It accounts for 1 in 160 live births. Trisomy is the most common aneuploidy disorder. Some of the common diseases are listed below.

  • Down’s syndrome- Trisomy of 21st chromosome
  • Edward’s syndrome- Trisomy of 18th chromosome
  • Patau syndrome- Trisomy of 13th chromosome
  • Klinefelter’s syndrome- There is an additional number of the X chromosome, XXY.
  • Turner’s Syndrome- One X chromosome is less, XO

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Frequently Asked Questions

What are aneugens?

Aneugens are agents that can cause aneuploidy. Most mutagenic carcinogens are covered under this term. Examples include X-rays, pesticides, chemicals like colchicine, etc.

What are tetrasomy and pentasomy?

The presence of four or five copies of a chromosome is termed tetrasomy and pentasomy respectively. It is common in sex chromosomes or allosomes. Examples include chromosomal disorders like XXXX, XXXXY, XXYY, XYYYY and XXXXX.

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