Disorders that result from an incorrect number of or structurally altered chromosomes
- Always present at birth but effects may not become apparent until later
- Gender as a risk factor depends on the type
- Lifestyle is not a significant factor
About 1 in 150–200 babies is born with a chromosomal abnormality. Some of these 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, sperm, and red blood cells) has 46 chromosomes arranged in 22 pairs plus the two 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. Humans have about 20,000–25,000 pairs of genes on these chromosomes, providing instructions for making proteins and for the growth, multiplication, and functioning of the body’s cells.
Chromosomal abnormalities are usually due to an error 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 result in 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?
A chromosomal abnormality involves either an incorrect number of chromosomes or a change 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 than those involving the sex chromosomes.
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 be associated with male secondary sexual characteristics not developing fully unless treatment with the male sex hormone testosterone is given. About 1 in 2,500 girls is born with only one X chromosome instead of the usual two, a condition known as Turner’s syndrome. Girls with this condition have short stature and, if they are not treated, fail to develop normal secondary sexual characteristics at puberty.
There is a natural exchange of genetic material between two chromosomes of a particular pair during meiosis. This mixing ensures that the genetic make-up of each egg or sperm is slightly different. Occasionally, errors occur during this process, resulting in a structural chromosomal abnormality. A small section of a chromosome may be deleted, duplicated, or inserted the wrong way around (inverted). These types of structural abnormalities affecting a chromosome may result in miscarriage or birth defects, ranging from mild to extremely severe. The effect on the fetus depends on the amount of genetic material that is altered and the chromosome affected. 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 problems as a result of inheriting too much or too little chromosomal material.
In this condition, a person has some body cells that contain a normal set of chromosomes and other cells that contain abnormal chromosomes. Mosaicism occurs when there is an error in cell division in the embryo soon after fertilization, resulting in a population of cells with abnormal chromosomes. These cells can often, but not always, be detected by analysis of a blood sample. The effects of mosaicism depend on the proportion and distribution of cells containing abnormal chromosomes.
How are they diagnosed?
Many chromosome disorders are obvious either at birth or once symptoms appear. The diagnosis can be confirmed by a blood test.
If a fetus dies in the womb, is stillborn, or is miscarried, it may be possible to test for chromosome defects. During a pregnancy, chromosomal abnormalities can be detected by taking a sample of fetal cells and analysing the chromosomes (see Antenatal genetic tests). Antenatal genetic tests may be offered to women at increased risk of having a baby with a chromosomal abnormality due to advanced maternal age or when Down’s syndrome in a relative is known to be caused by an inherited chromosomal abnormality. Certain routine tests or scans carried out in early pregnancy may also indicate an increased risk. In such cases, women may then be offered specific antenatal tests, such as amniocentesis or chorionic villus sampling, which can confirm the diagnosis of Down’s syndrome in a fetus.
What is the treatment?
There is no cure for disorders caused by chromosomal abnormalities. However, surgery in infancy may rectify physical problems, such as intestinal abnormalities or heart defects, and some people with sex chromosome disorders may be treated with hormone replacement. For example, girls with Turner’s syndrome can be given hormones to induce puberty and increase height.
When there is a history of chromosomal abnormality within the family or if recurrent miscarriages have occurred, prospective parents may wish to consider genetic counselling so that they can assess the risk of a child developing a chromosome disorder before deciding to begin a pregnancy.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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