Almost every cell in our body contains a cell nucleus that contains the 23 pairs of chromosomes that make up our genetic material. Each chromosome contains the DNA (deoxyribonucleic acid) that makes up hundreds of thousands of genes. Many of these genes produce proteins that play multiple roles in the human body. Some proteins are structural and make up our tissues, such as bones and muscles. Proteins called enzymes are involved in chemical reactions, such as the breakdown of the foods we eat. While others are like little messengers sending signals into our bodies, these proteins are called hormones.
All humans share the same set of genes, but the precise DNA sequence of these genes differs slightly from one individual to another (from about 0.1 to 0.2%). Although these differences take into account factors such as the different colors of the hair, eyes, and skin, they can also cause genetic diseases or susceptibility to disease. A disease-causing change in the DNA of a gene is called a mutation.
Types of genetic tests
- Diagnostic tests: This is used to confirm a diagnosis when a doctor sees a patient with symptoms suggestive of a particular genetic condition. For example, a person with movement disorders may be tested for Huntington’s disease. The genetic information obtained by the physician is useful for the treatment, management, and genetic counseling of the patient.
- Genetic screening tests: Some genetic tests are used even when the symptoms of a disease are not visible, but genetic information can help predict whether the person is at risk of developing or is susceptible to a particular disease.
- Prenatal screening for genetic diseases is offered to many women during pregnancy. Detection of Down syndrome in women over 35 is a good example. Screening for Down syndrome is usually done by amniocentesis or chorionic villus sampling at 14-20 weeks of pregnancy.
- Newborn screening is common in most hospitals around the world. It consists of screening for various disorders, including phenylketonuria, cystic fibrosis (CF). A blood sample is taken from the newborn. This blood sample is then sent to a laboratory for analysis.
- Carrier screening is used in people or populations to determine if they carry a mutation (called carrier) that may not affect the health of the person but may also affect the health of their future children. These mutations are usually recessive when both copies of the disease gene have to be mutated to cause the disease. In the case of an autosomal recessive disease, the child of a carrier will be affected by the disease only if his partner is also a carrier. Carrier screening is offered to prospective parents so that they can test whether they carry diseases such as cystic fibrosis (CF).
Before a genetic test is done, a doctor will perform a clinical examination and obtain a detailed family history. This will help the doctor to determine which gene may be responsible for the disease in question. The patient will be referred to a genetic counselor who can inform him about everything involved in genetic testing. The genetic counselor can explain what a particular genetic change means and how it affects you or your family.
Almost all genetic tests require a DNA sample from the patient, which is usually obtained with either a blood sample or a mouthwash (oral swab). This is then taken to a genetic test lab for analysis.
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