Our genes contribute to our development as people, including our risk of having medical conditions.
We all inherit genes from our parents. These genes are a set of building blocks for our growth and development. They determine things like our blood type, eye colour, hair and what our personalities will be like. Our genes can also cause or increase the risk of developing some medical conditions.
There are some medical conditions that are caused by genes, but most are caused by a combination of genes and environmental (life) factors.
Whether your child has a medical condition will depend on several factors, including:
- what genes they inherit
- whether the gene for that condition is dominant (you only need one copy of the faulty gene to have the condition) or recessive (you need two copies of the faulty gene, if you only have one, you will be a carrier but you will not have the condition)
- their environment, including any treatment they may get to prevent it.
Our genes are made up of DNA. Genetic mutations occur when DNA changes, altering the genetic instructions. Some medical conditions are directly caused by a mutation in a single gene that may have been passed on to a child by his or her parents.
These genetic conditions can be inherited in three main ways:
Autosomal recessive inheritance
Some conditions can only be inherited in an autosomal recessive pattern. This means the condition can only be passed on to a child if both parents have a copy of the faulty gene – both are 'carriers' of the condition.
If the child only inherits one copy of the faulty gene, they'll be a carrier but won't have the condition.
If a mother and a father both carry the faulty gene, there's a 1 in 4 (25%) chance of each child they have inheriting the genetic condition, and a 1 in 2 chance (50%) of their child being a carrier.
Examples of genetic conditions inherited in this way include:
- cystic fibrosis – a condition in which the lungs and digestive system become clogged with thick, sticky mucus
- sickle cell anaemia – a condition where red blood cells, which carry oxygen around the body, develop abnormally
- thalassaemia – a group of conditions where the part of the blood known as haemoglobin is abnormal, which means affected red blood cells are unable to function normally
- Tay-Sachs disease – a condition that causes progressive damage to the nervous system
Autosomal dominant inheritance
Some conditions are inherited in an autosomal dominant pattern. In this case, only one parent needs to carry the mutation for the condition to be passed on to the child.
If one parent has the mutation, there's a 1 in 2 (50%) chance it will be passed on to each child the couple has.
Examples of genetic conditions inherited in this way include:
- type 1 neurofibromatosis – a condition that causes tumours to grow along the nerves
- tuberous sclerosis – a condition that causes mainly non-cancerous (benign) tumours to develop in different parts of the body
- Huntingdon's disease – a condition where certain brain cells become increasingly damaged over time
- autosomal dominant polycystic kidney disease (ADPKD) – a condition that causes small, fluid-filled sacs called cysts to develop in the kidneys
Some conditions are caused by a mutation on the X chromosome (one of the sex chromosomes). These are usually inherited in a recessive pattern, but in a slightly different way from the autosomal recessive pattern described above.
X-linked recessive conditions often don't affect females much because they have two X chromosomes, one of which will almost certainly be normal and can usually make up for the changed chromosome. However, females who inherit the changed chromosome will become carriers.
Males can't inherit X-linked mutations from their fathers because they receive a Y chromosome from them. A male will develop the condition if he inherits it from his mother. This is because he won't have a normal X chromosome to compensate as a female would.
When a mother is a carrier of an X-linked mutation, each daughter they have has a 1 in 2 (50%) chance of becoming a carrier, and each son they have has a 2 in 2 (50%) chance of getting the condition from them.
When a father has an X-linked condition, his sons won't be affected because he'll pass on a Y chromosome to them. However, any daughters he has will become carriers.
Examples of genetic conditions inherited in this way include:
- Duchenne muscular dystrophy – a condition that causes the muscles to gradually weaken, resulting in an increasing level of disability
- haemophilia – a condition that affects the blood's ability to clot
- fragile X syndrome – a condition that usually causes certain facial and bodily characteristics, such as a long face, large ears and flexible joints
New genetic mutations
Although genetic conditions are often inherited, this isn't always the case.
Some genetic mutations can occur for the first time when a sperm or egg is made, when a sperm fertilises an egg, or when cells are dividing after fertilisation. This is known as a de novo, or sporadic, mutation.
Someone with a new mutation won't have a family history of a condition, but they may be at risk of passing it on to their children.
They may also have, or be at risk of developing, a form of the condition themselves.
Examples include some types of muscular dystrophy, haemophilia and type 1 neurofibromatosis.
Some conditions aren't caused by a mutation on a specific gene, but by an abnormality in a person's chromosomes, such as having too many or too few chromosomes, rather than the normal 23 pairs.
Examples of conditions caused by chromosomal abnormalities include:
- Down's syndrome – caused by having an extra copy of chromosome 21
- Edwards' syndrome – caused by having an extra copy of chromosome 18
- Patau's syndrome – caused by having an extra copy of chromosome 13
- Turner syndrome – a disorder that only affects females and is caused by a missing or abnormal X chromosome
- Klinefelter's syndrome – a disorder that only affects males and is caused by an extra X chromosome
While these are genetic conditions, they're generally not inherited. They usually occur randomly as a result of a problem before, during or soon after the fertilisation of an egg by a sperm.
How our environment impacts on our health
Very few health conditions are only caused by genes – most are caused by the combination of genes and environmental factors. Environmental factors include lifestyle factors, such as diet and exercise.
Around a dozen or so genes determine most human characteristics, such as height and the likelihood of developing common conditions.
Genes can have many variants, and studies of the whole genome – the whole set of genes – in large numbers of people are showing these variants may increase or decrease a person's chance of having certain conditions.
Each variant may only increase or decrease the chance of a condition very slightly, but this can add up across several genes.
In most people, the gene variants balance out to give an average risk for most conditions. But in some cases the risk is significantly above or below the average.
It's thought it may be possible to reduce the risk by changing environmental and lifestyle factors.
For example, coronary heart disease – when the heart's blood supply is blocked or interrupted – can run in families, but a poor diet, smoking and a lack of exercise can also increase your risk of getting the condition.
A consanguineous relationship is a union between two people who are blood relatives. It is more common in cultures from the Middle East, parts of Asia and North Africa. The most common form is marriage between first cousins.
Most babies born to couples who are blood relatives are healthy but it does increase the risk of a birth defect from 3% to 6%. Couples who are first cousins may double their risk of having a baby with a life-threatening birth defect. Children born to parents who were not first cousins but were closely related also had an increased risk.
Consanguinity is a common cultural practice in many societies around the world, particularly in the Middle East, parts of Asia and North Africa, as well as among emigrants from these communities now residing in North America, Europe and Australia.
There is a blood test you and the baby’s father can have to find out if you are carriers of a gene for sickle cell or thalassaemia. Anyone can ask their GP or local specialist sickle cell and thalassaemia centre for a free test at any time. You can find your nearest sickle cell and thalassaemia centre here.
If you or your partner are concerned you may be a carrier it's a good idea to get tested before you start a family.
Genetic testing can be used to find out whether a person is carrying a specific altered gene (genetic mutation ) that causes a particular medical condition.
It may be carried out for a number of reasons, including to:
- diagnose a person with a genetic condition
- help work out the chances of a person developing a particular condition
- determine whether a person is a carrier of a certain genetic mutation that could be inherited by any children they have
You'll usually need a referral from your GP or a specialist doctor for genetic testing to be carried out. Speak to your doctor about the possibility of testing if you think you may need it.
What does genetic testing involve?
Genetic testing usually involves having a sample of your blood or tissue taken. The sample will contain cells containing your DNA.
It can be tested to find out whether you're carrying a certain mutation and are at risk of developing a particular genetic condition.
In some cases, genetic testing can be carried out to find out whether a baby is likely to be born with a certain genetic condition.
This is done by testing samples of the fluid that surrounds the foetus in the womb (amniotic fluid) or cells that develop into the placenta (chorionic villi cells), which are extracted from the mother's womb using a needle.
Depending on the condition(s) being checked for, the fluid or cell samples will be examined and tested in a genetics laboratory to look for a specific gene, a certain mutation on a specific gene, or any mutation on a specific gene.
In some cases, it may be necessary to check an entire gene for mutations using a process called gene sequencing. This has to be done very carefully, and can take a long time compared with most other hospital laboratory tests.
Depending on the specific mutation being tested for, it can take weeks or even months for the results of genetic tests to become available. This is because the laboratory may have to gather information to help them interpret what's been found.
It isn't always possible to give definite answers after genetic testing. Sometimes it's necessary to wait to see if the person being tested, or their relatives, do or don't develop a condition. Other tests may need to be performed.
If your doctor thinks genetic testing may help you, you'll usually be referred for genetic counselling as well.
Genetic counselling is a service that provides support, information and advice about genetic conditions.
It's done by healthcare professionals who've received training in the science of human genetics (a genetic counsellor or a clinical geneticist).
What happens during genetic counselling will depend on exactly why you've been referred.
It may involve:
- learning about a health condition that runs in your family, how it's inherited, and which family members may be affected
- an assessment of the risk of you and your partner passing an inherited condition on to your child
- a look at the medical history of your family or your partner's family and drawing up a family tree
- support and advice if you have a child affected by an inherited condition and you want to have another child
- a discussion about genetic tests, which can be arranged if appropriate, including the risks, benefits and limitations of genetic testing
- help understanding the results of genetic tests and what they mean
- information about relevant patient support groups
You'll be given clear, accurate information so you can decide what's best for you.
Your appointment will usually take place at your nearest NHS regional genetics centre. The British Society for Genetic Medicine has details for each of the genetics centres in the UK.
Pre-implantation genetic diagnosis
For couples at risk of having a child with a serious genetic condition, pre-implantation genetic diagnosis (PGD) may be an option.
PGD involves using in-vitro fertilisation (IVF), where eggs are removed from a woman's ovaries before being fertilised with sperm in a laboratory.
After a few days, the resulting embryos can be tested for a particular genetic mutation and at most two of two unaffected embryos are transferred into the uterus.
PGD avoids having to terminate/abort foetuses affected by serious conditions, but it also has the lower success rate of getting pregnancy with IVF, and the costs of the both the IVF and PGD process, which isn't always available on the NHS.