What is Genetic Testing? The Debate Surrounding Genetic Testing Types of Genetic Testing The Science - How does Genetic Testing Work? Case Study: Karyotypes Biotechnology and Genetic Testing Genetic Tests Available in Canada Case Study: Phenylketonuria (PKU) Genetic Tests Available in Canada Bibliography

What is Genetic Testing?

Genetic testing, also known as gene testing and DNA tests, involves the examination of an individual's DNA. DNA makes up the genes which determine characteristics in living organisms, such as eye colour and height. Genetic testing detects the presence, absence or alteration of a particular gene, chromosome or protein through a variety of laboratory tools.

The Debate Surrounding Genetic Testing

Laboratory expertise in genetic testing has developed faster than the expertise in genetic counselling services, and studies in the effectiveness and value of genetic testing. This has raised numerous issues surrounding the usefulness of genetic tests and their risks, benefits and impact on society. Some of the issues are:

Genetic Privacy Confidentiality Prenatal Testing & Eugenics Genetic Testing & Insurance Standardization of Genetic Tests Certification of Laboratories

Genetic Discrimination Genetic Testing & Employment Informed Consent Genetic Counselling Services Quality Control

Types of Genetic Testing

Gene testing is used under several circumstances today. Currently, genetic tests are classified in health care according to their purpose.

• Genetic Screening - These tests are conducted on populations to identify individuals who are at risk of developing a specific disorder. Screening is done so that further testing can be undertaken. For example, women may be tested for BRCA1&2 - genes associated with breast cancer, so that preventative measures and early intervention can be considered.
  
  
• Carrier Testing - These tests are often conducted to determine if an individual is a "carrier" of a gene for a recessive genetic disorder. For example, couples undergo carrier testing for disorders such as Tay-Sachs disease, to assist in their reproductive decisions.
  
  
• Prenatal Diagnostic Testing - These tests are conducted before birth to determine whether a fetus is affected by, or at risk for, a genetic disorder. Down's Syndrome is the most common genetic disease screened by this method.
  
  
• Newborn Screening - These tests focus on the identification of metabolic disorders in newborns. Early detection and treatment may be crucial to reduce the progression of such diseases. One example is the newborn screening for phenylketonuria (PKU).
  
  
• Pre-symptomatic Testing - These tests are conducted on healthy individuals to determine whether or not they carry a genetic mutation that increases their likelihood of developing late-onset diseases and disorders. Examples include Huntington's disease and Alzheimer's disease.
  
  
• Susceptibility Testing - These tests are conducted to identify individuals with genetic mutations that make them more susceptible to developing a disease when exposed to certain environmental elements. This type of testing is often used to identify workers who may be susceptible to toxic substances that are found in their workplace which may cause disabilities.
  
  
• Diagnostic Testing - These tests are conducted on individuals to confirm a diagnosis already made by other methods. For example, genetic testing is often used to confirm the diagnosis of certain forms of cystic fibrosis (CF).
  
  
• Forensic/Identity Testing - These tests are conducted to discover genetic linkages in criminal investigations between suspects and evidence or between children and their biological parents.

The Science - How does Genetic Testing Work?

In genetic testing, scientists analyse samples (blood, amniotic fluid, etc.) from individuals for the presence of a particular gene or gene irregularities. These include mutated gene sequences, chromosomal abnormalities, atypical metabolite levels and unusually high or low protein levels.

The genes and gene irregularities are detected, measured and analysed through several different laboratory techniques. The techniques vary widely from the staining of entire chromosomes to the use of complementary DNA or antibodies to detect distinct gene sequences that are associated with a given disease or condition. Some of these techniques include:

• Gel electrophoresis
• Karyotyping
• DNA probes
• Radioactive probes
• Southern blotting
• Northern blotting
• Western blotting
• Autoradiography
• Polymerase Chain Reaction (PCR)

Case Study: Karyotypes

Karyotypes are pictures of all of our DNA, which contains 100,000 genes. It shows metaphase chromosomes of an individual cell, arranged in pairs and sorted according to size. There are 22 pairs of non-sex chromosomes and a pair of sex chromosomes. A karyotype provides the following information:

• the number of chromosomes per cell;
• the composition of the sex chromosomes; and
• identification of any chromosomal abnormalities.

Karyotypes are often performed during an amniocentesis which provides genetic information about the unborn fetus. This prenatal diagnostic screening can be used to detect fetal abnormalities such as Down's Syndrome and Turner's Syndrome.

Biotechnology and Genetic Testing

The development of laboratory expertise in genetic testing is occurring at a staggering pace. This is largely due to the decoding of the human genome and the discovery of gene sequences with links to certain diseases. The actual gene tests themselves use some of the biochemical laboratory techniques which have used in the study of biology for many years, especially in specialized areas like biochemistry and molecular biology.

Genetic Tests Available in Canada Case Study: Phenylketonuria (PKU)

Genetic tests, such as phenylketonuria (PKU) tests, have been used regularly in North America since the 1960s, allowing for early intervention and prevention of PKU. PKU tests are performed within 24 hours of birth. It involves taking a blood sample from a newborn and performing a Guthrie test. This determines the level of phenylalanine in the blood. Elevated levels indicate that the newborn has PKU. Children with PKU do not have an enzyme that breaks down phenylalanine, a type of amino acid found in various foods. Excessive levels of phenylalanine in the body causes mental retardation, which is avoided with diet control. The ultimate result of early detection is a considerable decrease in mental retardation caused by PKU.

Today, numerous genetic tests for different diseases are available in Canada through research and service laboratories. The following table lists examples of diseases for which genetic tests are available.

Genetic Tests Available in Canada

Type of Test	Diseases Tested
Genetic Screening	Breast Cancer Genes: BRCA1&2
	Phenylketonuria (PKU)
Carrier Testing	Lou Gehrig's Disease
	Tay Sach's Disease
Prenatal Diagnostic Testing	Down's Syndrome
	Spina Bifida
Newborn Screening	Phenylketonuria (PKU)
	Hypothyroidism
Pre-symptomatic Testing	Alzheimer's Disease
	Huntington Diesease
Susceptibility Testing	Chemical Metabolism Genes (CYP 2D6)
	P450 Metabolism Genes
Diagnostic Testing	Angelman Syndrome
	Cystic Fibrosis
Forensic/Identity Testing	DNA Fingerprinting
	Paternity Tests

As scientists continue to discover the links between diseases and genes, the number of genetic tests available in Canada will rise.

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