Aneuploidy: Definition & Disorders

Aneuploidy: Definition & Disorders
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  • 0:05 What Is Aneuploidy?
  • 1:38 How Does Aneuploidy Happen?
  • 3:20 Monosomy Disorders
  • 3:40 Trisomy Disorders
  • 4:52 Trisomy in Sex Chromosomes
  • 6:31 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson will define aneuploidy, what it is and what it means. The lesson will also identify some of the disorders associated with aneuploidy and how they come about.

What Is Aneuploidy?

'Two's company; three's a crowd.' Have you ever heard anyone use this expression? It usually refers to when someone's being a 'third wheel,' but it also ties nicely into something known as aneuploidy.

To understand aneuploidy, it's first necessary to know a bit about chromosomes. Ever seen the results of a trash compactor or car crusher? It takes big items and squishes everything into a nice little cube. Chromosomes are like the compacted form of genetic information and they are found in the nucleus of the cell. Human cells typically have 46 total chromosomes. They are passed from parents to child, with each parent contributing one set of 23 chromosomes.

Chromosomes have unique patterns, vary in size, and are often assigned numbers (except sex chromosomes, which are designated X and Y). They like to pair up (two's company) with their counterparts: chromosome 1 from Parent A pairs with chromosome 1 from Parent B. The pairs are termed homologous chromosomes, or homologs. Shown is a picture of all 46 chromosomes arranged by size, shape and number. This is called a karyotype. These karyotypes can tell scientists when something is different from the usual with a person's chromosomes.

Karyotype of a human male.
Typical human male karyotype

Aneuploidy is the loss or gain of a chromosome. Instead of pairs being formed, a single chromosome ends up alone or with multiple partners. You can think of aneuploidy as when you go on a date only to discover you've been stood up, or they've brought an extra, unwanted friend (three's a crowd).

How Does Aneuploidy Happen?

There are two main types of cells in your body. One type, germ cells, divides and give rise to cells with 23 chromosomes. The other type, somatic cells, gives rise to cells with 46 chromosomes.

There are many factors and proteins involved when cells divide. Sometimes factors or proteins are missing, or proteins don't function correctly. Occasionally when this happens, homologs, which are duplicated during cell division, don't separate correctly. This incorrect separation means the new cells don't have the typical number of chromosomes.

The incorrect separation often results in aneuploidy, but can be more drastic. Cells that have double the homologs (92 chromosomes) are called tetraploids, and those with many pairs of homologs are known as polyploids.

Aneuploidy can occur in both somatic and germ cells. The presence of aneuploid cells mixed with cells that do not exhibit aneuploidy creates a situation known as mosaic. In fact, mosaic aneuploidy can be found in the brain and liver where both tissues are healthy. However, aneuploidy has been found in cancerous cells, and there is debate about whether it is involved in causing cancer, is a result of cancerous factors, or both. Aneuploidy also alters cell division and growth, and therefore fetal development, often resulting in miscarriage or severe birth defects.

When germ cells exhibit aneuploidy, they give rise to certain disorders, depending on if a chromosome is missing, monosomy, or if there is an extra chromosome, trisomy. One thing to keep in mind is that aneuploidy is only rarely an inherited disorder.

Monosomy Disorders

The only survivable monosomy in humans is Monosomy X, or Turner's syndrome. In this syndrome, a female has only one X chromosome instead of two. Turner's occurs about 1 in every 2,000 births. The most common attribute is a short stature, and most children live to adulthood.

Trisomy Disorders

Remember 'three's a crowd'? There are several known trisomy disorders that involve chromosomes 13, 18, 21 and the sex chromosomes.

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