Also in this week's column:

• What is the difference between a chromosome and a gene?
• Which comes first: imagination or fantasy, and what's the difference between the two?

What are chromosome abnormalities and how often do they occur?

Asked by Lynn Davis of Casper, Wyoming

Each of our chromosomes has a specific and proper structure and colour. Together, they form a distinct pattern called the human karyotype. We have 46 chromosomes (23 from each parent). But biology being biology, there are variations in reproduction resulting in abnormalities in the karyotype of an individual. A very few are beneficial, most are harmless, a few are catastrophic.

A major chromosome abnormality (MCA) accounts for half of all spontaneous human abortions. A MCA occurs in about one in every 100 to 200 births. Recent medical diagnostic techniques can now detect many MCAs long before the child is born. They do this by analysing the patient's blood under a microscope. MCAs take one of five forms.

1. In duplication, instead of the normal two, the individual has three copies of the chromosome in every cell of their body (trisomic). Most trisomics result in a spontaneous abortion - but not always. Trisomy-21 (Down's Syndrome) occurs 1.5 times in every 1,000 births. The individual has three copies of chromosome 21. Trisomy-18 (Edward's Syndrome) occurs three times in every 10,000 births. The individual has three copies of chromosome 18. Trisomy-13 (Patau's Syndrome) occurs two times in every 10,000 births. The individual has three copies of chromosome 13.
2. In deletion, a piece of chromosome may go missing. The missing piece is usually from the end of the chromosome, but can be missing from the middle as well. The types of symptoms as well as their severity will vary depending upon the size and location of the deletion. Deletion syndromes occur at a frequency of about one in every 16,000 births. In Wolf-Hirschhorn Syndrome (occurring in one in every 50,000 births) the abnormality occurs on chromosome 4. In Cri-du-Chat Syndrome (occurring in one in every 50,000 births), the abnormality occurs on chromosome 5.
3. In translocation, a whole or a piece of one chromosome becomes attached to another.
4. In inversion, a portion of the chromosome breaks off, turns upside down, and reattaches.
5. In rings, a portion of the chromosome breaks off and forms a circle, with or without loss of genetic information. For these last three forms, there is an alteration in the pattern of genetic information.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au