Chromosome abnormality

Chromosome abnormalities, as the name suggests, are genetic disorders affecting the chromosome.

Error creating thumbnail:

Single Chromosome Mutations

Types

There are different types of chromosome abnormalities: Numerical or structural.

Numerical chromosome abnormalities

A numerical abnormality mean an individual is either missing one of the chromosomes from a pair or has more than two chromosomes instead of a pair.

Error creating thumbnail:

Numerical chromosome aberration

An example of a condition caused by numerical abnormalities is Down syndrome, which is marked by mental retardation, learning difficulties, a characteristic facial appearance and poor muscle tone (hypotonia) in infancy. An individual with Down syndrome has three copies of chromosome 21 rather than two; for that reason, the condition is also known as Trisomy 21. An example of monosomy, in which an individual lacks a chromosome, is Turner syndrome. In Turner syndrome, a female is born with only one sex chromosome, an X, and is usually shorter than average and unable to have children, among other difficulties.

Error creating thumbnail:

Human chromosome diseases set

Structural abnormality

A structural abnormality means the chromosome's structure has been altered in one of several ways.

• Structural Abnormalities: A chromosome's structure can be altered in several ways.
  • Deletions: A portion of the chromosome is missing or deleted.
  • Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.
  • Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocation. 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.
  • Inversions: A portion of the chromosome has broken off, turned upside down, and reattached. As a result, 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.

Error creating thumbnail:

Schematic illustration of chromosomal aberrations

How does it happen?

Most chromosome abnormalities occur as an accident in the egg or sperm. In these cases, the abnormality is present in every cell of the body. Some abnormalities, however, happen after conception; then some cells have the abnormality and some do not.

Inheritance

Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual). This is why, when a child is found to have an abnormality, chromosome studies are often performed on the parents.

How do chromosome abnormalities happen?

Chromosome abnormalities usually occur when there is an error in cell division. There are two kinds of cell division, mitosis and meiosis.

Error creating thumbnail:

Screening and counseling for genetic conditions

• Mitosis results in two cells that are duplicates of the original cell. One cell with 46 chromosomes divides and becomes two cells with 46 chromosomes each. This kind of cell division occurs throughout the body, except in the reproductive organs. This is the way most of the cells that make up our body are made and replaced.
• Meiosis results in cells with half the number of chromosomes, 23, instead of the normal 46. This is the type of cell division that occurs in the reproductive organs, resulting in the eggs and sperm.

In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too many copies of a chromosome. Errors can also occur when the chromosomes are being duplicated.

Other factors that can increase the risk of chromosome abnormalities are:

• Maternal Age: Women are born with all the eggs they will ever have. Some researchers believe that errors can crop up in the eggs' genetic material as they age. Older women are at higher risk of giving birth to babies with chromosome abnormalities than younger women. Because men produce new sperm throughout their lives, paternal age does not increase risk of chromosome abnormalities.
• Environment: Although there is no conclusive evidence that specific environmental factors cause chromosome abnormalities, it is still possible that the environment may play a role in the occurrence of genetic errors.

See also

• Aneuploidy
• Chromosome segregation
• Genetic disorder
  • List of genetic disorders
• Gene therapy
• Nondisjunction
• Obstetrical complications

External links

• Chromosome+disorders at the US National Library of Medicine Medical Subject Headings (MeSH)