Genetics is the branch of science that deals with how you inherit physical and behavioural characteristics.
Your genes contain the genetic information that controls characteristics such as the colour of your eyes and hair.
Genes are single units of genetic material found inside chromosomes (long, threadlike structures in each of the body's cells that contain DNA, see below).
However, characteristics are not just due to genes alone – environment also plays an important role. The way a child is brought up will have a direct impact on their development.
For example, a child may inherit 'tall genes' from their parents, but if their diet does not provide them with necessary nutrients, they may not grow very tall.
Read more about how genes are inherited.
Each cell in the body contains 23 pairs of chromosomes. These carry the genes you inherit from your parents.
As well as inheriting characteristics such as eye and hair colour, you can also inherit a health condition, or a tendency to develop a particular condition.
One of each pair of chromosomes is inherited from each parent, which means that with one exception, there are two copies of each gene in each cell.
The exception is with the X and Y sex chromosomes. These determine the sex of a baby. Babies with an X and a Y chromosome will be male (XY). Those without a Y chromosome, but two of the same kind of X chromosome, will be female (XX). This means that males only have one copy of each X chromosome gene.
Each chromosome contains hundreds or thousands of genes. There are about 21,000 genes in total, all written into the DNA contained in the chromosomes. The whole set of genes is known as the genome.
Deoxyribonucleic acid (DNA) is the long molecule that stores genetic information. This information is written in the DNA using an alphabet of four 'letters', or smaller molecules: A, C, G and T.
Each gene consists of a precise sequence of letters. This is the genetic code. Each gene contains the instructions to make a particular protein, in a particular cell, at a particular time. Proteins are complex chemicals that make up:
- tendons (fibrous cords that join bones to muscle)
DNA is made of two strands coiled together, each one a mirror image of the other. Because of this, DNA can be easily split when a cell divides and the genetic code is passed on exactly. This DNA copying process when a cell divides is known as replication.
There are around 3 billion letters of DNA code in the 23 chromosomes.
The Human Genome Project
The Human Genome Project is an international scientific project that involves thousands of scientists around the world.
The initial project ran from 1990 to 2003. Its objective was to map the immense amount of genetic information found in every human cell.
As well as identifying specific human genes, the Human Genome Project has enabled scientists to gain a better understanding of how certain traits and characteristics are passed on from parents to children.
It has also led to a better understanding of the role of genetics in inherited conditions such as:
- muscular dystrophy – a condition that gradually causes the muscles to weaken over time, leading to an increasing level of disability
- Down's syndrome – a condition that affects a child's normal physical development and causes learning difficulties
- cystic fibrosis – a condition that causes internal organs to become clogged with thick, sticky mucus
Genetic testing and counselling
Genetic testing can be used to find out whether you are carrying a particular genetic mutation (altered gene) that causes a medical condition.
Testing usually involves taking a blood or tissue sample. The sample will contain cells containing your DNA. It can be tested to find out whether you are carrying a particular mutation and are at risk of developing a particular genetic condition.
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. There may be different forms of the same gene. These different forms are called alleles.
For example, for the gene that determines eye colour, there may be an allele for blue eyes and an allele for brown eyes.
You may inherit a brown allele from your mother and a blue allele from your father. In this instance, you will end up with brown eyes because brown is the dominant allele. The different forms of genes are caused by mutations (changes) 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 factors including:
- what genes they inherit
- whether the gene for that condition is dominant or recessive
- their environment, including any 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 do not occur spontaneously. DNA is constantly being damaged by normal processes and natural chemicals, including water and oxygen, as well as radiation and sunlight.
The damage is usually repaired, but occasionally the repair may not be perfect. This can lead to a mistake being made when the DNA is copied while a cell divides, causing a mutation.
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 will lead to lung cancer.
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 does not work, which may cause disease
Mutations can be inherited from a parent or they can occur when a sperm or egg is made, resulting in a new mutation.
Someone with a new mutation will not 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 partial or modified form of the condition themselves.
Conditions often caused by new genetic mutations include:
- Duchenne muscular dystrophy – the most common and most severe form of muscular dystrophy (a condition that gradually causes muscles to weaken)
- haemophilia – a condition that affects the blood’s ability to clot
Passing on mutations
Eggs and sperm only contain one of each pair of chromosomes present in the adult. Which one of each pair of chromosomes goes into an egg or sperm is random.
Therefore, if a mutation is present in only one of the two copies of a gene (one of a pair of chromosomes), there is a one in two chance of it being passed on to a child.
In recessive inheritance, a child inherits a mutation in both copies of a particular gene. In other words, both parents must have a copy of the faulty gene (they are ‘carriers’ of the condition) to pass on the condition. If the child only inherits one copy of the faulty gene, they will be a carrier of the condition.
If a mother and a father both carry the faulty gene, there is a one in four chance their child will have the genetic condition.
For example, a recessive gene causes cystic fibrosis. This means a child with cystic fibrosis has inherited a faulty copy of the gene from both their mother and their father.
In dominant inheritance, a mutation only needs to be passed on from either the mother or the father. So, if one of the parents has the condition, there is a one in two chance it will be passed on to the child.
A dominant gene causes type 1 neurofibromatosis, a condition that can cause tumours to grow on nerves throughout the body. A child can inherit the condition if either their mother or father has it and passes the faulty gene on. It can also be caused by a new genetic mutation.
If there is a mutation in a gene on the X chromosome, the effect may not be seen in females. This is because females have two X chromosomes, one of which is almost certainly normal.
However, if a male inherits the mutation on the X chromosome from his mother, he will not have a normal copy of the gene and will develop the condition. Both Duchenne muscular dystrophy and haemophilia are inherited in this way.
Very few health conditions are only caused by genes; most are caused by the combination of genes and environment. The environment includes lifestyle factors, such as diet and exercise.
Around a dozen or so genes determine most human characteristics, such as height and the likelihood of catching common diseases.
Genes can have many variants (alleles – see above). Studies of the whole genome (the whole set of genes) in large numbers of individuals are showing that these variants may increase or decrease a person’s chance of having a condition. 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 is thought that it may be possible to reduce the risk by changing environmental and lifestyle factors.
While your genes may increase your likelihood of developing a certain condition, this risk is also closely linked to lifestyle factors. These factors are all linked, because a gene or genes may also make you sensitive to something in the environment.
For example, coronary heart disease (when the heart's blood supply is blocked) 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 it will be possible for individuals to find out what conditions they are most likely to develop. It may then be possible for you to avoid these conditions by making lifestyle and environmental changes.
Genetic testing and counselling
Genetic testing can be used to find out whether a person is carrying a specific genetic mutation (altered gene) that causes a particular medical condition.
Read more about genetic inheritance and genetic mutations.
Genetic testing usually involves having a sample of your blood or tissue taken. The sample will contain cells that contain your DNA and can be tested to find out whether you are carrying a particular mutation and are at risk of developing a particular genetic condition.
Genetic testing is only useful if it is known that a specific genetic mutation causes a condition. For example, a specific gene is known to cause spinal muscular atrophy (a condition where there is muscle weakness and progressive loss of movement). It is therefore possible to test a blood sample for the presence or absence of this gene.
Some genetic conditions are caused by particular mutations. Others can be caused by any mutation in a gene. For example, cystic fibrosis is usually caused by a few particular mutations, which means it is relatively easy to test for.
However, Marfan syndrome (a condition that affects the body's connective tissues) can be caused by any one of hundreds of different possible mutations in a particular gene.
For a condition such as Marfan syndrome, a scientist in a laboratory will have to check the entire gene for mutations, using a process called gene sequencing. This has to be done very carefully, and it can take a long time compared to most other hospital laboratory tests.
For example, the Marfan syndrome gene contains more than 200,000 letters of DNA code, divided into 65 parts. And, this is not even the largest human gene.
Even when a mutation is found, much work may still be needed to determine whether or not it is actually the cause of the condition.
Clinical geneticists are doctors trained in genetics and skilled at providing advice about genetic conditions. Many conditions can be caused by more than one gene and can be inherited in different ways.
Determining the cause and pattern of inheritance is important. This is because it makes it easier to advise someone about their choices if a firm diagnosis has been made.
Doctors and hospital doctors may make referrals to genetics services, often at the request of people with a genetic condition or their families.
Many other specialists also use genetics services, including:
- obstetricians (specialists in the care of women during pregnancy) – they may send a sample of fluid from the womb of a pregnant woman for chromosome tests
- paediatricians (specialists in conditions that affect children) and neurologists (nervous system specialists) – they often seek advice about diagnosis and genetic tests
- oncologists (specialists in cancer) and surgeons – they may refer people or their families for genetic tests if they are at increased risk of getting cancer
Genetics is increasingly becoming a part of everyday medicine.
Genetic counselling is a service that provides information and advice about genetic conditions.
Counselling is conducted by healthcare professionals who have been specially trained in the science of human genetics (a genetic counsellor or a clinical geneticist).
The counsellor will discuss the risks, benefits and limitations of genetic testing with you. They will also explain how the information found as a result of genetic testing could have implications for both you and your family.
Genetics services often help people with genetic conditions who want to have children. For example, if someone has an inherited condition and wants to become a parent, genetic counselling can assess the risk of passing the condition on to the child.
Your family history is usually very important. Identifying family members who have had the condition in the past will help determine which genetic tests may be appropriate.
Geneticists will be able to explain the results of any tests or examinations that you have and help you decide how to progress.
A geneticist can also help assess your risk of developing a particular condition. For example, if you have a strong family history of cancer, a geneticist will assess your risks and discuss them with you. They can help you decide whether to have cancer screening or other tests.