Genetic disorders

The human race has a huge diversity in many features – skin colour, size, intellectual and athletic abilities, to name just a few. This variation has arisen largely due to:

  • changes in the DNA determining these features
  • changes to and interactions with the environment.

Any change in our genes or DNA is called a mutation. All genetic variation has arisen from mutations. The different forms of a gene that arise through mutation are called alleles.

While we most often associate mutations with genetic diseases, much of the variation has led to a highly complex body that works extremely well in our environment.

Some mutations may change the gene so that it codes for a protein that works just as well, or maybe even better than, the protein coded for by the original gene.

Unfortunately, however, some gene changes result in the production of a different protein that does not work as efficiently, or in the same manner, as the one usually coded for by that gene. In some cases, no functional protein is produced at all. In such cases, the mutation or gene change may cause a genetic condition or disease, such as cystic fibrosis or Huntington's disease.

Approximately 3% of Australian babies may be born with a genetic condition where one or more genes that played an important role have been mutated.

Cystic fibrosis - a case study

Cystic fibrosis is a hereditary disease which affects around 1 in every 2,500 Caucasian babies. Its occurrence in African-Americans is much lower - about one in every 17,000 births. It affects several organs in the body, causing thick, sticky mucous secretions in the lungs and pancreas. This leads to respiratory problems, including recurrent infections, and difficulty digesting food.

Everyone has two copies of the cystic fibrosis gene. For every child affected by cystic fibrosis, there are around 100 people who carry the faulty gene. Being a carrier means that one copy of the gene is faulty, and the other is normal. Carriers are not affected by cystic fibrosis because the normal gene compensates for the faulty one. When both copies are faulty, cystic fibrosis results.

If both parents carry the gene responsible for the disease, they have a one-in-four chance of having an affected child.

This diagram illustrates how genes for diseases such as cystic fibrosis can be passed from parents to their children. If two carriers of the CF gene have a child, there is a:

  • 1 in 4 chance that the child will have cystic fibrosis
  • 2 in 4 chance that it will be a carrier like its parents
  • 1 in 4 chance that it will not carry the gene at all.

Where there is a risk that genes for diseases may be passed on, it is important to note that the figures above apply to every child independently. For example, if a couple has three healthy children, it does not mean that the fourth child will have cystic fibrosis.