Genetic disorders
There is huge diversity in many features found in the human race – 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, plus changes to and interactions with the environment.
Any change in our genes or DNA is called a mutation and 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 disease.
Most babies in Australia are born healthy. However approximately 3% may be born with a genetic condition where one or more genes that played an important role have been mutated.
For fact sheets on how genetic disorders are inherited, go to the Centre for Genetics Education. There are fact sheets on:
Autosomal recessiveness: http://www.genetics.com.au/pdf/factSheets/FS6.pdf
Autosomal dominance: http://www.genetics.com.au/pdf/factSheets/FS7.pdf
X-linked inheritance: http://www.genetics.com.au/pdf/factSheets/FS8.pdf
For more information on genetic disorders:
http://www.genetics.com.au
http://www.genetics.com.au/schools/ethicscases.htm# samplesforcasestudies
http://www.genecrc.org
Cystic fibrosis - a case study
Cystic fibrosis is an hereditary disease which affects around 1 in every 2,500 babies born. 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.
For every child affected by cystic fibrosis, there are around 100 people who carry the faulty gene. Everyone has two copies of the gene which, when both copies are faulty, causes cystic fibrosis. 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.
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 like 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, a 2 in 4 chance that it will be a carrier like its parents, and a 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. For example, if a couple has three healthy children, it does not mean that the fourth child will have cystic fibrosis.
Your Genes, Your Health looks at various aspects of cystic fibrosis (and other genetic disorders), from causes and treatments to what it is like to live with the disease on a daily basis.