Genetics is the branch of science that deals with how you inherit physical and behavioural characteristics, including medical conditions.
Your genes are a set of instructions for the growth and development of every cell in your body.
For example, they determine characteristics such as your blood group and the colour of your eyes and hair.
However, many characteristics aren't the result of genes alone - environment also plays an important role.
For example, children may inherit "tall" genes from their parents, but if their diet doesn't provide them with the necessary nutrients, they may not grow very tall.
Genes are packaged in bundles called chromosomes. In humans, each cell in the body contains 23 pairs of chromosomes - 46 in total.
You inherit one of each pair of chromosomes from your mother and one from your father. This means there are two copies of every gene in each cell, with the exception of the sex chromosomes, X and Y.
The X and Y chromosomes determine the biological sex of a baby. Babies with a Y chromosome (XY) will be male, and those without a Y chromosome will be female (XX).
This means that males only have one copy of each X chromosome gene, rather than two, and they have a few genes found only on the Y chromosome that play an important role in male development.
Occasionally, individuals inherit more than one sex chromosome. Females with three X chromosomes (XXX) and males with an extra Y (XYY) are normal, and most never know they have an extra chromosome.
The whole set of genes is known as the genome. Humans have about 21,000 genes on their 23 chromosomes, plus 37 genes in their mitochondria.
Mitochondria are small structures inside cells that allow the cell to get energy from sugar and fat, and they're only passed on within eggs. This means mitochondrial DNA is only inherited from mothers.
Genes are made up of deoxyribonucleic acid (DNA). DNA is a long molecule made up of a combination of four chemicals: adenine, thymine, cytosine and guanine, represented as letters A, T, C and G.
These "letters" are ordered in particular sequences within your genes. They contain the instructions to make a particular protein, in a particular cell, at a particular time.
Proteins are complex chemicals that are the building blocks of the body. For example, keratin is the protein in hair and nails, while haemoglobin is the red protein in blood.
Genes and medical conditions
As well as determining characteristics such as eye and hair colour, your genes can also directly cause or increase your risk of developing a wide range of medical conditions.
Although not always the case, many of these conditions occur when a child inherits a specific altered (mutated) version of a particular gene from one or both of their parents.
Examples of conditions directly caused by genetic mutations include:
- muscular dystrophy - causes the muscles to weaken over time, leading to an increasing level of disability
- Down's syndrome - affects a child's normal physical development and causes learning difficulties
- cystic fibrosis - causes the lungs and digestive system to become clogged with thick, sticky mucus
- Lynch syndrome - causes an increase in the risk of certain cancers; for example, bowel cancer and womb cancer
There are also many conditions that aren't directly caused by genetic mutations.
These conditions can occur as the result of a combination of an inherited genetic susceptibility and environmental factors, such as a poor diet, smoking and a lack of exercise.
Read more about how genes are inherited.
Genetic testing can be used to find out whether you're carrying a particular genetic mutation that causes a medical condition.
This can be useful for a number of purposes, including:
- diagnosing certain genetic conditions
- predicting your likelihood of developing a certain condition
- determining if any children you have are at risk of developing an inherited condition
Testing usually involves taking a blood or tissue sample and analysing the DNA in your cells.
Genetic testing can also be used to find out whether a foetus is likely to be born with a certain genetic condition. A sample of cells from the womb is extracted and tested.
Read more about genetic testing and counselling.
Each cell in the body contains 23 pairs of chromosomes. One chromosome from each pair is inherited from your mother and one is inherited from your father.
The chromosomes contain the genes you inherit from your parents.
For example, for the gene that determines eye colour, you may inherit a brown-eye gene from your mother and a blue-eye gene from your father.
In this instance, you'll end up with brown eyes because brown is the dominant gene. The different forms of genes for eye colour are caused by changes (mutations) in the DNA code.
The same is true for medical conditions. There may be a faulty version of a gene that results in a medical condition, and a normal version that may not cause health problems.
Whether your child ends up with a medical condition will depend on several factors, including:
- what genes they inherit
- whether the gene for that condition is dominant or recessive
- their environment, including any preventative treatment they may receive
Genetic mutations occur when DNA changes, altering the genetic instructions. This may result in a genetic disorder or a change in characteristics.
Mutations can be caused by exposure to specific chemicals or radiation. For example, cigarette smoke is full of chemicals that attack and damage DNA.
This causes mutations in lung cell genes, including the ones that control growth. In time, this can lead to lung cancer.
Mutations can also occur when DNA fails to be copied accurately when a cell divides.
Mutations can have three different effects. They may:
- be neutral and have no effect
- improve a protein and be beneficial
- result in a protein that doesn't work, which may cause disease
Passing on mutations
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.
Depending on the specific condition concerned, these genetic conditions can be inherited in three main ways, outlined below.
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 of the condition but won't have the condition.
If a mother and a father both carry the faulty gene, there's a one in four (25%) chance of each child they have inheriting the genetic condition, and a one in two 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 one in two (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 to a significant degree because they have two X chromosomes, one of which will almost certainly be normal and can usually compensate for the mutated chromosome. However, females who inherit the mutation 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 the mutation from his mother. This is because he doesn't have the normal X chromosome to compensate.
When a mother is a carrier of an X-linked mutation, each daughter they have has a one in two (50%) chance of becoming a carrier, and each son they have has a one in two (50%) chance of inheriting the condition.
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 of the mutation.
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
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 the mutation on to their children.
They may also have, or be at risk of developing, a form of the condition themselves.
Examples of conditions that are often caused by a de novo mutation 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.
Influence of the environment
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 developing the condition.
Research suggests that in the future, individuals will be able to find out what conditions they're most likely to develop.
It may then be possible to significantly reduce the chances of developing these conditions by making appropriate lifestyle and environmental changes.
Genetic testing and counselling
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.
You can find out more about genetic testing and how it's carried out by reading What happens in a genetics laboratory? (PDF, 1.90Mb).
If your doctor thinks genetic testing may be appropriate for 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 conducted 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.