Introduction

Genetics
Genetics

Genetics is the branch of science that deals with how you inherit physical and behavioural characteristics – including medical conditions.

About genes

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.

Chromosomes

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. That is females with three X chromosomes (XXX) and males with an extra Y (XYY). Most will never know they have an extra chromosome. However, females with one X have a condition known as Turner syndrome, and males with an extra X have Klinefelter syndrome.

The whole set of genes is known as the genome. Humans have about 21,000 genes on their 23 chromosomes.

DNA

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 and 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 – which causes the muscles to weaken over time, leading to an increasing level of disability
  • Down's syndrome – which affects a child's normal physical development and causes learning difficulties
  • cystic fibrosis – which causes the lungs and digestive system to become clogged with thick sticky mucus

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

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.

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 huge 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 a number of genetic and inherited conditions.

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Facts

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 – called alleles.

For example, for the gene that determines eye colour, 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 several factors, including:

  • what genes they inherit
  • whether the gene for that condition is dominant or recessive (see below)
  • their environment, including any preventative treatment they may receive

Genetic mutations

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 does not 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 onto a child by his or her parents. These are known as monogenic conditions.

Depending on the specific condition concerned, monogenic conditions can be inherited in three main ways. These are outlined below.

Autosomal recessive inheritance

For conditions that are inherited in an autosomal recessive pattern to be passed on to a child, both parents must have a copy of the faulty gene (they are ‘carriers’ of the condition).

If the child only inherits one copy of the faulty gene, they will be a carrier of the condition but will not have the condition themselves.

If a mother and a father both carry the faulty gene, there is a one in four (25%) chance of each child they have inheriting the genetic condition and a one in two chance (50%) of them 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

For conditions that are inherited in an autosomal dominant pattern to be passed on to a child, only one parent needs to carry the mutation.

If one parent has the mutation, there is 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:

X-linked inheritance

Some conditions are caused by a mutation on the X chromosome (one of the sex chromosomes). These are usually inherited in a recessive pattern – albeit in a slightly different way to the autosomal recessive pattern described above.

X-linked recessive conditions often don't affect females to a significant degree because females 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.

If a male inherits the mutation from his mother (males cannot inherit X-linked mutations from their fathers because they will receive a Y chromosome from them), he will not have a normal copy of the gene and will develop the condition.

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 will not be affected because he will 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

New mutations

Although genetic conditions are often inherited, this is not 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 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 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.

Chromosomal conditions

Some conditions are not 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 – a condition caused by having an extra copy of chromosome 21
  • Edwards' syndrome – a condition caused by having an extra copy of one chromosome 18
  • Turner syndrome – a condition that only affects females, caused by a missing or abnormal X chromosome
  • Klinefelter's syndrome – a condition that only affects males, caused by extra X chromosome

While these are genetic conditions, they are generally not inherited. Instead, they usually occur randomly as a result of a problem before, during, or soon after the fertilisation of an egg by a sperm.

Multi-factorial conditions

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 individuals are showing that 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 is thought that 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 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 significantly reduce the chances of developing these conditions by making appropriate lifestyle and environmental changes.

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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.

It may be carried out for a number of reasons, including:

  • diagnosing a person with a genetic condition
  • helping work out the chances of a person developing a particular condition
  • determining if a person is a carrier of a certain genetic mutation that could be inherited by any children they have

You will usually need to get a referral from your GP, or a specialist doctor if you have one, for genetic testing to be carried out – speak to your GP or 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 and can be tested to find out whether you are carrying a particular mutation and are at risk of developing a particular genetic condition.

In some cases, genetic testing can be carried out to see if a foetus is likely to be born with a certain genetic condition by testing samples of amniotic fluid (the fluid that surrounds the foetus in the womb) or chorionic villi cells (cells that develop into the placenta) extracted from the mother's womb using a needle.

Depending on the condition(s) being tested for, the blood or cell samples will then be tested and examined in a genetics laboratory to check 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 it can take a long time compared to 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 can be because the laboratory has to gather information to help them interpret what has been found.

It is also important to realise that it is not always possible to give definite answers after genetic testing. Sometimes it is necessary to wait to see if the person being tested or other relatives do, or do not develop a condition, and other tests may need to be performed.

You can find out more about genetic testing and how it is carried out by reading the leaflet: 'What happens in a genetics laboratory?' (PDF, 1.90Mb).

Genetic counselling

If your doctor thinks genetic testing may be appropriate in your case, you will usually be referred for genetic counselling as well.

Genetic counselling is a service that provides support, information and advice about genetic conditions. It is conducted by healthcare professionals who have been specially trained 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 in understanding the results of genetic tests and what they mean
  • information about relevant patient support groups

You will 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 Genetic Alliance UK 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 a maximum of two unaffected embryos are transferred into the uterus.

While PGD has the advantage of avoiding the termination of foetuses affected by serious conditions, it also has a number of drawbacks. These include the modest success rate of achieving a pregnancy after IVF, as well as the substantial financial (PGD is not always available on the NHS) and emotional burdens of the combined IVF and PGD process.

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The information on this page has been adapted by NHS Wales from original content supplied by NHS Choices.
Last Updated: 13/10/2016 09:23:04