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Chromosome disorders

AGE  Always present at birth but effects may not become apparent until laterGENDER  Risk factors depend on the type
LIFESTYLE  Not a significant factorGENETICS  

Chromosome disorders result from an incorrect number of or structurally altered chromosomes.

Chromosome disorders
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About 1 in 150-200 babies is born with a chromosomal abnormality. Some of these abnormalities do not affect health, but most may cause multiple problems, including physical abnormalities and learning difficulties. About 50-60 per cent of all miscarriages are the result of chromosome disorders. However, there are many chromosome disorders, such as Turner's syndrome, Klinefelter's syndrome, and Down's syndrome, that are not fatal.

Every cell in the body, apart from eggs and sperm, has 46 chromosomes arranged in 22 pairs plus the 2 sex chromosomes. One of each pair is inherited from each parent. The sex chromosomes determine an individual's gender. Females have two X chromosomes; males have one X and one Y chromosome. The other 22 pairs are known as autosomes. Each cell is estimated to contain about 35,000 pairs of genes on these chromosomes, providing instructions for making proteins involved in the growth, multiplication, and function of the body's cells.

Chromosomal abnormalities are usually due to a fault in the division of chromosomes that occurs when eggs and sperm are formed. This process, called meiosis, involves a halving of the number of chromosomes in normal body cells. Normal meiosis results in each sex cell having only 23 chromosomes. Occasionally, a chromosomal abnormality can arise early in the division of the fertilized egg. Chromosomal abnormalities usually have major physical and/or mental effects.

Several factors increase the likelihood of a couple having a child with a chromosomal abnormality, such as already having a child with a chromosome disorder or a maternal age of over 35 years.

What are the types of chromosome disorder?

A chromosomal abnormality involves either an incorrect number of chromosomes or a defect in the structure of a chromosome. These abnormalities can affect any of the 44 autosomal chromosomes or the sex chromosomes. Disorders that are caused by autosomal chromosome abnormalities are usually more severe.

Numerical chromosome abnormalities

Mistakes occasionally occur in the way chromosome pairs are divided between the new egg or sperm cells during meiosis, with one cell having too many chromosomes while the other has too few. For example, if an egg containing an extra chromosome is fertilized by a normal sperm, the embryo will have an extra chromosome in every cell in its body. If a sperm with a missing chromosome fertilizes a normal egg, the fetus will have one less chromosome in each of its cells. About two-thirds of all chromosome disorders are caused by cells that contain the wrong number of chromosomes.

Extra or missing autosomal chromosomes usually result in miscarriage of the embryo. One exception to this is having an extra chromosome 21, known as trisomy 21. Although fetuses with trisomy 21 are often miscarried, a number survive, and they have the condition known as Down's syndrome. Abnormalities in the number of sex chromosomes tend to have a less severe effect on the embryo and, in some cases, there are no obvious signs of a disorder. About 1 in 500 babies is born with an extra X or Y chromosome. An extra X chromosome in a girl or Y chromosome in a boy may have little or no physical effect. However, boys who are born with an extra X chromosome (XXY) have a disorder known as Klinefelter's syndrome, which may prevent normal male secondary sexual characteristics developing. About 1 in 2,500 girls is born with only one X chromosome instead of the usual two, a acondition known as Turner's syndrome. Girls with this condition have a short stature and, if they are not treated, fail to develop normal secondary sexual characteristics at puberty. Girls with Turner's syndrome are usually infertile.

Structural chromosome abnormalities

There is a natural exchange of genetic material between pairs of chromosomes during meiosis. This mixing ensures that the genetic make-up of each egg or sperm is slightly different. Occasionally, faults occur during this process, resulting in a structural chromosomal abnormality. A small section of chromosome may be deleted, duplicated, or inserted the wrong way around (inverted). These types of structural abnormalities in the chromosome may result in miscarriage or birth defects, ranging from mild to extremely severe. The effect on the fetus depends on the amount and type of genetic material that is altered. Material may also be exchanged between two different chromosomes following breaks in each one. This process is known as translocation. If no genetic material is gained or lost during this process, it is known as a balanced translocation. Balanced translocations are carried by about 1 in 500 people and rarely cause health problems. However, the child of a person who carries a balanced translocation may have serious chromosomal abnormalities as a result of inheriting too much or too little chromosomal material.

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Posted 09.09.2010

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