Types of Chromosomal Abnormalities
The two most common types of chromosomal abnormalities are:
- Trisomy: Occurs when there is three copies, instead of the usual two, of a specific chromosome. Down syndrome is the most commonly known example of a trisomy. Down syndrome occurs when there are three copies of chromosome 21 present, which is why it is sometimes referred to as trisomy 21. The only other types of live trisomy births are Trisomy 18 (also known as Edward’s syndrome) and Trisomy 13 (also known as Patau’s syndrome). Both are very rare, occurring in fewer than 1 in 5,000 live births. Most patients do no survive for more than a month because the complications of both are so severe.
- Missing, or Extra, Sex Chromosomes: Some of the most common types of chromosomal abnormalities occur in the sex hormones. Normally, women have two X chromosomes and men have one X and one Y chromosome. About 1 in 2500 girls born will have Turner syndrome, which occurs when there is only one X chromosome pregnant. Girls with Turner may be shorter than their peers, and will not begin puberty unless they receive hormone injections. Turner syndrome does not result in any mental abnormalities. Klinefelter syndrome occurs in 1 in 600 to 800 boys, and is the result of having two, or more, X chromosomes, in addition to a Y chromosome. Boys with Klinefelter are usually tall and of average intelligence, but may have trouble with judgment and impulse control. Like girls with Turner syndrome, boys with Klinefelter are sterile as adults and have lower than normal levels of testosterone, which can be corrected with hormone injections.
Less common types of chromosomal abnormalities include:
- Cri-Du-Chat (Cat Cry) Syndrome: This is caused by a deletion of chromosome 5. Symptoms include: high-pitched cry in infancy (sounding like a cat), physical abnormalities and mental retardation.
- Prader Willi Syndrome: This is caused by a missing piece of chromosome 15. Symptoms include: extreme obesity, mental retardation and other issues.
- Deletions from Chromosome 22: This can cause cleft palates, heart defects, velocardiofacial and DiGeorge syndromes.