Genetic Information

Understanding Genetics

Our bodies contain millions of cells which contain our genetic information. Within each cell there is a nucleus. Within each nucleus there are 46 chromosomes arranged in pairs (23 pairs in total). The 23rd pair are the sex chromosomes.

There are two chromosomes in each numbered pair because an individuals always inherits one copy from the egg from their mother, and one copy from the sperm of their father. Individuals with two X chromosomes are female while individuals who have an X and a Y chromosome are male.

Normal Male
46 XY

Normal Female
46 XX
You should only have two in each pair. If an individual has a different number than two chromosomes in each pair they have a chromosome abnormality. The most commonly talked about chromosome abnormality is Down syndrome, also known as trisomy 21, since the condition primarily is caused by the presence of an extra chromosome 21.

When an individual has a structural rearrangement on one of the chromosomes they also have a type of chromosome abnormality. Two common types of structural chromosomal rearrangements include a translocation and an inversion.

A translocation is where a piece of one chromosome breaks off and attaches to another chromosome. An inversion is where a particular segment of a chromosome has inverted itself.

46,XY,t(5;8)(q15;q24.1) (Translocation)

This is an example of a ‘balanced’ structural constitutional chromosome abnormality. Part of chromosomes 5 and 8 have broken and exchanged. These patients are usually phenotypically normal, but have an increased risk of miscarriages and abnormal offspring.

In addition to chromosome conditions there also exists genetic conditions. A genetic condition affects a particular gene and not the entire chromosome. We have thousands of genes located throughout each chromosome, each with their own unique function. The genes that are situated on chromosome 1 are different than the genes found on chromosome 21. When there is a change in the gene that affects its structure or function we call that a mutation. Some mutations are harmless, other cause significant physical and/or mental impairments.

It is helpful to seek genetic counselling when there is a known familial mutation to talk about options for testing, potential risks for future future pregnancies and for family members. It is also helpful to discuss effective coping mechanisms for dealing with the issues associated with having a genetic disorder within the family (i.e., caring for an affected relative, understanding a genetic diagnosis, dealing with at-risk status, understanding reproductive choices, risks for stigmatization, insurance issues).

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September 26, 2006