What are DNA, genes and chromosomes?

Your body is made up of millions of tiny cells. Different types of cells form the different structures of the body, including skin, muscles, nerves and also organs such as the liver and kidneys.

This image was derived from Eukaryote DNA.svg, via Wikimedia Commons

In the centre (nucleus) of most cells in your body, the DNA molecule is packaged into thread-like structures called chromosomes. You have 46 chromosomes arranged in 23 pairs. These include one pair of sex chromosomes (either XX for females and XY for males). The other chromosomes that do not determine whether we are male or female are called autosomes. There are 22 pairs of autosomes (numbered 1 to 22). One chromosome from each pair comes from your mother and one from your father.

A gene is the basic unit of your genetic material. It is made up of a sequence (or piece) of DNA and sits at a particular place on a chromosome. So, a gene is a small section of a chromosome. Each gene controls a particular feature or has a particular function in your body. For example, dictating your eye colour or hair colour, making all the various proteins in your body, etc. Each gene is part of a pair. One gene from each pair is inherited from your mother, the other from your father. Each chromosome carries hundreds of genes. Humans have between 20,000 and 25,000 genes altogether. The total of all your genes is called your genome.

DNA stands for deoxyribonucleic acid. DNA forms your genetic material. Genes, which are made up of DNA, act as instructions to make proteins. In humans, genes vary in size from just a very small amount of DNA to very large amounts of DNA.

Proteins are large, complicated molecules that play many important roles in your body. They do most of the work in cells and are required for the structure, function and regulation of your body's tissues and organs.

As our cells are multiplying all the time, our genetic information needs to stay the same. Normally, there are excellent mechanisms in place to make sure each cell gets the exact same copy of DNA, the material that makes up our genes. However, sometimes the copying mechanism makes mistakes or other problems can occur with your genetic material. Problems and abnormalities in genes can lead to genetic diseases.

Genetic testing is a type of medical test that identifies changes in chromosomes, genes or proteins. Gene tests look for abnormalities in DNA taken from a person's blood, body fluids or tissues. The tests can look for large mistakes such as a gene that has a section missing or added. Other tests look for small changes within the DNA. Other mistakes that can be found include genes that are too active, genes that are turned off, or those that are lost entirely.

Genetic tests examine a person's DNA in a variety of ways. They are all designed to identify differences between the gene being tested and what would be considered to be a normal version of the same gene.

There are different types of genetic testing which include:

These look at single genes or short lengths of DNA taken from a person's blood or other body fluids (for example, saliva) to identify large changes, such as:

An example of a genetic disorder that is tested in this way is cystic fibrosis.

However, there are limitations to genetic testing, as it is only useful if it is known that a specific genetic mutation causes a certain condition. A mutation or error in copying the DNA results in a permanent change to the DNA which can result in a number of diseases. For example, a specific gene mutation is known to cause Huntington's disease. It is therefore possible to test a blood sample for the presence or absence of this gene mutation. For many conditions - for example, diabetes - there may be any one of hundreds or even thousands of different possible mutations in a particular gene. This means genetic testing for those conditions is virtually impossible.

These look at the features of a person's chromosomes, including their structure, number and arrangement. Parts of a chromosme can be missing, be extra or even be moved to a different part on another chromosome.

There are different ways in which chromosome tests can be undertaken. These include:

Biochemical tests look at the amounts or activities of key proteins. As genes contain the DNA code for making proteins, abnormal amounts or activities of proteins can signal genes that are not working normally. These types of tests are often used for newborn baby screening. For example, biochemical screening can detect infants who have a condition affecting one of the many essential chemical reactions in the body (metabolic condition) such as phenylketonuria.

Genetic test results can confirm or rule out a suspected genetic condition or help determine a person's chance of developing or passing on a genetic disorder. More than 2,000 genetic tests are currently in use, and more are being developed all the time.

Genetic testing is performed in different ways including:

Newborn screening is done just after birth to identify genetic disorders that can be treated early in life. For example, every baby in the UK is tested for cystic fibrosis as part of the heel prick test.

Diagnostic testing is used to identify or rule out a specific genetic disorder if a baby or person has symptoms to suggest a certain genetic disorder (for example, Down's syndrome).

Carrier testing is used to identify people who carry one copy of a gene mutation (a genetic change) that, when present in two copies, causes a genetic disorder (for example, sickle cell disease). This type of test can be useful to provide information about a couple's risk of having a child with a genetic disorder.

Before birth (prenatal) testing is used to detect changes in an unborn baby's genes. This type of testing is offered during pregnancy if there is an increased risk that the baby will have a genetic or chromosomal disorder. It cannot identify all possible inherited disorders and birth defects, however.

Pre-implantation genetic testing is available for couples who are at risk of having a child with a specific genetic or chromosome disorder, eg cystic fibrosis, sickle cell disease or Huntington's disease.

Egg cells are removed from the woman's ovaries and then fertilised with sperm cells outside the body. This is called in-vitro fertilisation (or IVF). The eggs are fertilised with sperm cells to form embryos. The fertilised embryos develop for three days and then one or two cells are removed from each embryo.

The genetic material (DNA and chromosomes) from the cells are tested for the known disorder in the family history. One or two of the unaffected embryos are then transferred into the mother's womb (uterus). If the pregnancy is successful, the baby will not be affected by the disorder it was tested for.

Predictive testing is used to detect genetic mutations associated with disorders that appear after birth, often later in life. These tests can be helpful to people who have a family member with a genetic disorder but who have no features of the disorder themselves at the time of testing (for example, breast cancer associated with the BRCA1 gene). Predictive testing can identify mutations that increase a person's risk of developing disorders with a genetic basis, such as certain types of cancer.

Testing can also determine whether a person will develop a genetic disorder, such as haemochromatosis, before any signs or symptoms appear. People in families at high risk for a genetic disease have to live with uncertainty about their future and their children's future.

A genetic test result showing a known gene mutation responsible for a certain disease as not being present in a person can provide a sense of relief. However a positive result may have a devastating effect on a person's life, especially if there is no known treatment.

However for some disorders a positive result may help you to consider options to prevent the disorder. For example, women with BRAC1 are at increased risk of breast cancer and may decide to have surgery to remove their breasts (mastectomy) or to take a medicine called tamoxifen to reduce the risk. See the separate leaflet on Breast Cancer for more information.

Therefore before having predictive testing it is essential for a specialist to carefully discuss with you your risks of being affected by the disorder, how the disorder would affect you and the benefits and risks of having a genetic test for the disorder. See the section on genetic counselling below.

Forensic testing uses DNA sequences to identify a person for legal purposes. Unlike the tests described above, forensic testing is not used to detect gene mutations associated with disease. This type of testing can also be used to work out the paternity of a child. Forensic testing can also be used for identifying human remains when identification is not possible by other means - for example, after a natural disaster such as a fire or tsunami.

Genetic testing usually involves taking a sample of blood or tissue. In adults and children this usually involves taking a blood sample from a vein. Some genetic tests can be done from samples of saliva or from taking a sample (swab) from the inside of your mouth.

In pregnancy, a sample may be taken from the baby by amniocentesis or chorionic villus sampling. In amniocentesis a sample of the liquid (amniotic fluid) that surrounds a baby is taken. It is done by putting a needle though the tummy (abdomen) into the womb (uterus). In chorionic villus testing a sample of part of the placenta is taken. This is either done by inserting a needle into the abdomen like in amniocentesis or by putting a thin tube into the neck of the womb (cervix). Both tests involve a very small risk that you may have a miscarriage as a result of having the test. If you are offered these tests, doctors will discuss the risks involved to help you to make a choice about whether to have the test or not.

In recent years the Harmony test has become available. This can be used during pregnancy and is done using a sample of the mother's blood, so there is no risk of miscarriage as there is with amniocentesis or chorionic villus sampling.

In newborns, routine screening for genetic disorder such as phenylketonuria happens as part of a baby's heel prick test when they around 5 days old.

After the sample has been taken it is sent to the laboratory for testing.

It may take anywhere from weeks to months for the results of all the tests to come back. This depends on the type of genetic test you've had. Your doctor should advise you how long the results will be.

A variety of genetic tests can be bought individually, many now over the Internet, which usually involve scaping the inside of your cheek to obtain some cells for testing. These are not recommended by doctors. Many test for genetic disorders for which there is no treatment, so they can heighten anxieties if you test positive for one of these disorders. They may also test for diseases that you may never actually develop in the future if you do not have other risk factors. For example, testing positive for the BRAC1 gene does not mean that you will definitely develop breast cancer in the future.

Before you undergo any of these tests, it may be worth asking yourself if you are prepared to make changes in your lifestyle, based on the test results. If you are not willing to take actions like stopping smoking or exercising more, such tests may not be of much benefit to you.

Many of these tests are also unreliable and can lead to very misleading results. If you would like to be tested for a genetic disorder then you should talk to your doctor about this in more detail.

The information obtained from genetic testing can have a profound impact on your life so you may be referred to a genetic counsellor Genetic counselling is available to anyone undergoing, or thinking of undergoing, any form of genetic testing. Genetic counselling is not a psychological therapy. It aims to provide you with all the information you need to make a decision about whether you should have a genetic test.

Genetic counselling may include information about:

The information is given in a way that will allow you to make your own decision. Only you can decide what is right for you. The counselling is essential to make sure you have all the important information you need to make the decision.

As they consider the options available to them, people are influenced by:

Post-test counselling is also available to help you deal with the results of the test.

More:
Genetic Testing: How It Works, Types, and Diagnosis | Patient

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