Genetic Testing 2016


GENETIC TESTING

What is Genetic Testing?

Genetic testing is a medical test performed on blood and other tissue. It is mostly used to detect changes in genetic material (chromosomes or genes) that may result in chromosomal abnormalities or genetic conditions. Genetic testing can also be performed to determine paternity, obtain information about genealogy (ancestry) or for forensic investigation.

What are the major reasons for genetic testing?

Diagnostic testing

Diagnostic testing is used to confirm a diagnosis, made during a medical examination/assessment, of a suspected genetic condition or chromosomal abnormality. The result from a diagnostic test is used to guide future health care, to predict the risk of the condition for other children, future pregnancies and other family members.  

Prenatal testing

Prenatal testing is performed during pregnancy to determine the chance (risk) for the baby to be born with a birth defect (congenital) or a genetic condition. There are two types of prenatal tests, screening and diagnostic.  Screening tests identify pregnancies at high risk for a birth defect or chromosomal abnormality.   Diagnostic tests confirm the presence of a specific birth defect or genetic condition in the pregnancy. These tests cannot test for ALL genetic conditions and birth defects.  For more detailed information go to Prenatal Genetic Counselling.

Predictive testing

  • Predictive testing is performed on someone who does not have symptoms but the results determine whether there is a risk for a genetic condition in the future.  There are two types of predictive tests, presymptomatic and susceptibility.
  • Pre-symptomatic predictive testing defines with certainty if a genetic condition will develop, such as Huntington disease and Familial Adenomatous Polyposis (a type of colon cancer)
  • Susceptibility predictive testing defines if a predisposition to develop a genetic condition exists but cannot confirm whether it will occur. Other factors such as genes and/or environmental factors play a role in the development of the condition, such as hereditary breast cancer.

Carrier testing

Carrier testing is performed to identify individuals who have one copy of a gene fault (mutation), that when present in two copies, causes a genetic disorder. Carrier testing benefits people with a family history of a genetic disorder or are from an ethnic group known to have an increased risk for specific genetic conditions. The test result can be used to define the risk of a genetic condition for children, future pregnancies and other family members.

Preimplantation testing

Preimplantation testing or preimplantation genetic diagnosis (PGD), is a specialised prenatal test used to reduce the risk of a child born with a particular genetic or chromosomal disorder. It is offered to couples where there is an increased risk for such a condition. It differs from prenatal diagnostic testing as the test is performed on an embryo created using reproductive techniques such as in-vitro fertilization, and prior to implantation.

PGD is performed on a few cells removed from the embryo. This test is used to detect the presence of specific chromosomal changes or a gene fault (for a genetic condition). Only embryos without these changes are implanted in the womb to initiate a pregnancy.


Comparison of two types of Genetic Tests: Gene Tests and Chromosome Tests

 

Gene Test Chromosome Test
  Function
  • Detect changes in the genetic material (including DNA and RNA) of a gene.
  • Detect changes in the number and structural appearance of the chromosomes
  Techniques or Methods
  • Mutation Detection : Identifying a known fault (mutation) in the gene.
  • DNA Sequencing: Identify an unknown fault (mutation) in a gene.
  • Karyotype: Using a microscope to view and analyse all the chromosomes in a cell
  • qPCR: Identify the presence of certain chromosomes.
  • FISH: Identify regions of certain chromosomes.
  Sample type and products used for testing
  • Blood sample
  • DNA extracted directly from the blood
  • Blood sample, amniotic fluid (amniocentesis), placenta cells (chorionic villus sampling)
  • Cells are grown and chromosomes viewed
Timing*
  • Known mutation (6-8 weeks)
  • Gene sequencing (Months)
  • Cell cultures for karyotype and FISH (4 weeks)
  • Quick test for certain chromosomes(72 hours)
Examples and Uses
  • Determine the presence of a genetic condition, such as cystic fibrosis.
  • Diagnostic, Prenatal, Carrier, Predictive and PGD testing.
  • Determine the presence of a chromosomal abnormality such as Down syndrome
  • Diagnostic, Prenatal and Carrier testing.


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