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Human genetics and genetic technologies
on this page
- » What is the human genome?
- » What is the relationship between DNA mutations and genetic conditions?
- » Why is genetic testing a potentially powerful tool in medicine?
- » Are consent and privacy issues in genetics any different to other areas of medical practice?
on other pages in this topic
- » Direct to Consumer (DTC) DNA tests
- » Genetic discrimination
- » How genetic contributions to common diseases are found – genetic association studies
- » Sequencing your genome
- » Use of genetic information in sport
- » Ethical issues in human genetics
What is the human genome?
A human genome is all the DNA contained in the nucleus, or core, of a human cell. DNA molecules are very long, so DNA is packaged up tightly into chromosomes. Human cells have 23 pairs of chromosomes – one chromosome of each pair is inherited from each parent. This means that siblings (brothers and sisters) have about half of their DNA in common, while more distant relatives have less DNA in common (first cousins share about one quarter of their DNA).
Humans have two kinds of chromosome: sex chromosomes (the X and Y chromosomes), and autosomes. There are 22 paired autosomes and two sex chromosomes in each cell. Females have two X chromosomes and males have one X chromosome and one Y chromosome.
Each chromosome is made up of a single DNA molecule. DNA contains the instructions for building different parts of a cell and is in the form of a chemical code (the genetic code). DNA is made up of chains of four basic building blocks known as nucleotide bases. These bases are Adenine (A), which is always paired with Guanine (G), and Thymine (T) which is always paired with Cytosine (C).
The human genome is made up of over 3 billion of these genetic bases. Groups of these bases form genes which function to instruct the cell to build other molecules such as proteins. There are between 20,000 and 25,000 genes in the human genome.
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What is the relationship between DNA mutations and genetic conditions?
A genetic disease or condition is caused by one or more gene changes, which scientists refer to as mutations. A mutation is a permanent change in the DNA code that makes up a gene. Mutations that can be passed on to children are usually inherited from a parent, but some happen for unknown reasons during foetal development. This type of inherited mutation is called a germ line mutation. Mutations also build up in a person’s DNA over their life time (for example, DNA damage due to sun exposure). These mutations are not passed on to children and are called somatic mutations.
Changes to the DNA code do not always lead to health problems. Some mutations occur in regions of the chromosome that do not encode genes. These mutations do not affect the function of genes, but can be used in research studies (for example, genetic association studies).
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Why is genetic testing a potentially powerful tool in medicine?
Mutations in the genes an individual is born with can result in genetic disorders that may cause problems at birth, in childhood, in adolescence, in adulthood or very late in life. Some gene changes do not cause health problems for the individual but may cause health problems for their children. Genetic testing allows genes to be examined for mutations. This information can be used to work out the chance of a disorder developing for that person in the future or the risk of a genetic disorder occurring in the person’s children.
Gene mutations that cause health problems are quite rare, so genetic tests are only recommended when there is a history of a genetic health problem in the family. Blood relatives share regions of their DNA with each other. If one member of a family is found to have a genetic mutation, other members of the family may also have the same mutation. If the mutation causes health problems, then the health of other family members could be at risk. Doctors usually encourage people to share this type of information with their family so that other members can seek medical advice as appropriate.
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Are consent and privacy issues in genetics any different to other areas of medical practice?
The 2003 Australian Law Reform Commission & Australian Health Ethics Committee enquiry into genetic privacy issues discusses various issues related to consent and privacy in the document Essentially Yours – the protection of human genetic information in Australia (see http://www.nhmrc.gov.au/about/committees/hgac/index.htm for this report as well as the government’s response). A recommendation from this enquiry was that it should be illegal to undertake DNA testing without proper consent unless directed to by the court. This would include paternity testing.
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