Genetics
Topic Overview
What are genes?
Genes are the part of a body cell that contains the biological information that parents pass to their children. Genes control the growth and development of cells. Genes are contained in DNA (deoxyribonucleic acid), a substance inside the centre (nucleus) of cells that contains instructions for the development of the cell.
You inherit half of your genetic information from your mother and the other half from your father. Genes, alone or in combination, determine what features (genetic traits) a person inherits from his or her parents, such as blood type, hair colour, eye colour, and other characteristics, including risks of developing certain diseases. Certain changes in genes or chromosomes may cause defects in various body processes or functions.
What are chromosomes?
Many genes together make up larger structures within the cell called chromosomes. Each cell normally contains 23 pairs of chromosomes.1
A human has 46 chromosomes (23 pairs). One chromosome from each pair comes from the mother, and one chromosome from each pair comes from the father. One of the 23 pairs determines your sex. The sex chromosomes are called X and Y.
- For a child to be female, she must inherit an X chromosome from each parent (XX).
- For a child to be male, he must inherit an X chromosome from his mother and a Y chromosome from his father (XY).
Some genetic disorders are caused when all or part of a chromosome is missing, or when an extra chromosome or chromosome fragment is present.
What is genetic testing?
Genetic testing examines a DNA sample for gene changes, or it may analyze the number, arrangement, and characteristics of the chromosomes. Testing may be performed on samples of blood, semen, urine, saliva, stool, body tissues, bone, or hair.
Should I have genetic testing?
You may choose to have genetic testing if you are concerned that you have an increased risk of having or getting a disease that has a genetic cause. The information you obtain from the tests may help you make decisions about your life. For example:
- If tests determine that you have an increased risk for passing on a disease to your child, you may choose to have more genetic testing while you are pregnant (prenatal testing). Or you may decide to adopt a child.
- If tests determine that you have an increased risk for developing a disease such as breast cancer, you may make decisions that help lower your risk for breast cancer.
- You may feel reassured if the tests are normal.
You may decide to have a genetic test during pregnancy to determine whether your fetus has a disorder, such as Down syndrome. Information obtained from the test can help you decide how to manage your pregnancy.
Genetic testing can be used to determine the identity of a child's father (paternity). It can also be used in crime scene investigation.
What are the main types of genetic testing?
There are five main types of genetic testing:
- Carrier identification
determines whether people who have a family history of a specific disease or who are in a group that has an increased chance for that disease are likely to pass on that disease to their children. Information obtained from this type of testing can help guide a couple as they make decisions about pregnancy.
- Prenatal testing determines whether a fetus has a disorder, such as Down syndrome. Information gained from this type of testing can help guide decisions about how to manage a pregnancy, including the decision about whether to end the pregnancy.
- Newborn screening checks for various metabolic diseases, such as phenylketonuria (PKU). Information obtained from newborn screening can help guide medical treatment to ensure the best possible outcome for the baby.
- Late-onset disease testing determines whether you carry a genetic change that increases your risk for developing a disease, such as breast cancer or Huntington's disease, later in life. This might be of interest if you have a relative with the disease. Information obtained from this type of testing can help you make decisions about preventing or managing the disease.
- Genetic identification (DNA fingerprinting) can be used to determine paternity, help solve crimes, and identify a body. DNA fingerprinting is more accurate than dental records, blood type, or traditional fingerprints.
What are the risks of genetic testing?
The information obtained from genetic testing can affect your life and the lives of your family members. The issues involved include:
- Psychological. The emotions you may experience if you learn that you have a greater chance of having or passing on a serious disease can cause you to feel anxious or depressed. This may also affect your relationship with your partner or other family members. Genetic counselling is recommended prior to genetic testing.
- Medical. A person who tests positive for a disease-specific gene may decide to use preventive or treatment options to reduce the impact or severity of the disease. Although many treatment options are proven effective, others may be potentially dangerous or of unproven value.
- Privacy. Genetic testing and counselling are generally covered by provincial health plans. Some people worry that genetic information released to insurance companies may affect future employment options or insurance availability. Results will not affect your health coverage under your provincial health plan.
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| Author: | Douglas Dana Maria G. Essig, MS, ELS | Last Updated: February 29, 2008 |
| Medical Review: | Anne C. Poinier, MD - Internal Medicine Siobhan M. Dolan, MD, MPH - Reproductive Genetics Renee H. Martin, PhD - Medical Genetics | |
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