These are variations from normal in the number or structure of chromosomes contained in a person’s cells. In most cases, the chromosomal abnormality is present in all of the body’s cells. The possible effects range from virtually none to a lethal condition, depending on the particular type of abnormality.
The cause of a chromosomal abnormality is generally a fault in the process of chromosome division, either during the formation of an egg or sperm, or during the first few divisions of a fertilized egg. Occasionally a parent passes on an abnormal arrangement of his or her own chromosomes.
Chromosomal abnormalities are classified according to whether they involve the 44 autosomes (pairs of very similar chromosomes) or the two sex chromosomes (X and Y). A whole extra set of chromosomes per cell is called polyploidy; this is lethal in early pregnancy.
These abnormalities cause physical and mental defects of varying severity. Some types of autosomal abnormality, known as trisomy, involve the presence of an extra chromosome on one of the 22 pairs of autosomes. The most common trisomy is Downs syndrome, which is caused by the presence of three number 21 chromosomes.
Sometimes, part of a chromosome is missing, as in cri du chat syndrome. In translocation, a part of a chromosome is joined to another, causing no ill effects in the person but a risk of abnormality in his or her children.
Sex Chromosome Abnormalities
Normally, a female has two X chromosome and a male has an X and a Y. Abnormalities may occur if there are missing or extra sex chromosomes.
In Turners’ syndrome, a girl is born with only a single X chromosome in her cells instead of the normal complement of two. The condition causes physical abnormalities, defective sexual development, and infertility.
In boys, one or more extra X chromosomes causes Klinefelter’s syndrome. This condition results in defective sexual development and infertility.
The presence of an extra X chromosome in women or an extra Y chromosome in men normally has no physical effect but increases the risk of mild learning difficulties.
If suspected, chromosomal abnormalities can be diagnosed by chromosome analysis in early pregnancy, using amniocentesis or chorionic villus sampling.
Because chromosomal abnormalities affect every one of a person’s cells, no cure is possible. Many disorders caused by autosomal chromosome defects result in early death. Others, such as Downs syndrome, are compatible with survival but cause physical and mental disability. Hormonal or surgical treatment, or a combination, can help to correct some of the development defects causes by Klinfelter’s and Turner’s syndromes.
Anyone who has a child or other family member affected by a chromosomal abnormality may wish to consider genetic counselling to establish the risk of his or her future children being affected by the condition.