Chromosomal Abnormalities

As women start having babies later in life, chromosomal abnormalities are becoming a greater concern. Adding to that is the availability of genetic screening, which may allow people to know if there is a problem before the birth of their child. However, it also carries the risk of false positives.

It is important to note that chromosomal abnormalities are not the same as genetic defects. A genetic defect or disease is the result of a specific gene that one or both parents carry, such as the gene for cystic fibrosis. A chromosomal disorder is the result of damage to the DNA instructions carried in an egg, or a sperm.

What is a Chromosome?
A chromosome is the structure inside a cell that consists of proteins and DNA. The word chromosome comes from the Greek words for color - chroma, and body - soma. Inside a chromosome is all of the genetic information and material that will direct a cell to develop into a part of the body. Information such as hair color, height and sex is determined by the contents of our chromosomes.

Usually, a person will have 46 chromosomes, ordered in 23 pairs. One half of each pair comes from our mother, and the other from our father. Any alterations or damage done to the number, or structure, of chromosomes will result in a chromosomal abnormality, which can have a serious impact on physical and mental development.

Causes of Chromosomal Abnormalities
A chromosomal abnormality begins in the reproductive tissues of both parents. Eggs and sperm usually have 23 chromosome each, half of the parent’s chromosomes. Occasionally something will go wrong when the chromosome splits and a sperm or an egg can end up with missing pieces of DNA, or there might be extra genetic material. Usually there is no known cause for these abnormalities and therefore nothing can be done to prevent them.

When one of these damaged sperm or eggs join with a normal sperm or egg, the result will be an embryo with a chromosomal abnormality. Usually these embryos do not survive and a chemical pregnancy, or early miscarriage is the result, often without the woman even knowing she was pregnant. It is estimated that up to 70% of first trimester miscarriages are the result of chromosomal abnormalities.

Types of Chromosomal Abnormalities
The two most common types of chromosomal abnormalities are:

1. 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.
2. 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.