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Genetic Testing
We all have 46 chromosomes in our cells. Chromosomes are made from DNA. Genes are short
sections of DNA (deoxyribonucleic acid) and each chromosome contains hundreds to thousands of
genes. Genes contain the information our bodies need to make chemicals called proteins. Proteins
form the structure of our bodies and play an important role in the processes that keep us alive. It is
the differences in our genes that makes us all individuals. Sometimes a gene may change (this is
called a mutation) and either cause or increase the risk of a disease or disorder.
Genetic testing is a type of medical test that identifies changes in chromosomes, genes or proteins.
Genetic tests examine a person's DNA in a variety of ways. They are all designed to identify a
particular gene that may cause a genetic disorder.
Note: the information below is a general guide only. The arrangements, and the way tests are
performed, may vary between different hospitals. Always follow the instructions given by your doctor
or local hospital.
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.
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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.
What is genetic testing?
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:
Molecular genetic tests (or gene tests)
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:
A gene that has part of it missing or a section added; or
Small changes, such as a missing, added or altered part within the DNA strand.
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.
Chromosomal genetic tests
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:
Karyotyping - this test results in a picture of all of a person's chromosomes. It can identify changes in
the number of chromosomes (for example, Down's syndrome in which there is an extra chromosome
number 21).
Fluorescent in situ hybridisation (FISH) analysis - this test looks at certain parts of the
chromosomes and can detect very small pieces of chromosomes that are either missing or extra (for
example, Duchenne muscular dystrophy).
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Biochemical tests
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.
Who has genetic testing performed on them?
The results of a genetic test 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
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
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
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.
Prenatal testing
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 testing
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. 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
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 BRAC1
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.
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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 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
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.
How is genetic testing done?
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 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 some time for the results of all
the tests to come back. Your doctor should advise you how long the results will be.
What are the risks of the home testing kits?
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.
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What about genetic counselling?
The information obtained from genetic testing can have a profound impact on your life. 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 implications of testing positive for the genetic disorder, including the psychological impact and
other consequences.
Whether to inform relatives of your intention to test, or not to test.
The usual pattern of progression of the disease you are being tested for and its potential treatments.
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 information you need to make the decision.
As they consider the options available to them, people are influenced by:
The risk of transmitting a disorder.
The severity of a disorder.
The availability of diagnosis before birth (prenatally).
Moral, social and religious convictions.
Post-test counselling is also available to help you deal with the results of the test.
Further help & information
Genetic Alliance UK
Unit 4D, Leroy House, 436 Essex Road, London, N1 3QP
Tel: 0207 704 3141
Web: www.geneticalliance.org.uk
Further reading & references
Promoting Gene Testing; UK Genetic Testing Network, 2013
Cirulli ET; The Increasing Importance of Gene-Based Analyses. PLoS Genet. 2016 Apr 7;12(4):e1005852. doi:
10.1371/journal.pgen.1005852. eCollection 2016 Apr.
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical
conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its
accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions.
For details see our conditions.
Original Author:
Dr Rachel Hoad-Robson
Current Version:
Dr Colin Tidy
Peer Reviewer:
Prof Cathy Jackson
Document ID:
12711 (v5)
Last Checked:
30/11/2016
Next Review:
30/11/2019
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