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Page last reviewed: 13/07/2011

Genetics is the branch of science that deals with how you inherit your physical and behavioural characteristics. The genetic information that controls these characteristics, such as the colour of your hair and eyes, is located in genes (single units of genetic material), which are found in chromosomes.

However, it is important to remember that characteristics are not just due to genes alone - the environment is very important.

A child may inherit tall genes from their parents, but if the child is malnourished (has a diet that does not give them the nutrients they need to maintain good health), they may not grow very tall.


Each cell in the body contains 23 pairs of chromosomes. These carry the genes that you inherit from your parents. In this way you can inherit a health condition or disease, or a tendency to develop a particular condition. One in each pair is inherited from each parent, so with one exception, there are two copies of each gene in each cell. 

The exception is with the X and Y chromosomes, which determine your sex (see the box to the left). People with an X and a Y chromosome develop as males. Those without a Y chromosome develop as females. This means that males only have one copy of each X chromosome gene.

Each chromosome contains hundreds or thousands of genes. In total, there are about 21,000 genes, all written into the DNA contained in the chromosomes. The whole set of genes is called the genome.


Deoxyribonucleic acid (DNA) is the long molecule that stores genetic information. This information is written in the DNA using an alphabet of only 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 the:

  • blood
  • muscles
  • tendons (fibrous cords that join bones to muscle)
  • skin
  • nerves
  • organs

DNA is made of two strands coiled together, each one a mirror image of the other (see the image to the left). Because of this, it can be divided easily when a cell divides and the genetic code is passed on exactly. The process of DNA copying when a cell divides is called replication.  

There are about 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 that is 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 resulted in 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 baby's normal physical development and causes mild to moderate learning difficulties
  • cystic fibrosis - a condition that causes the internal organs to become clogged with thick, sticky mucus
Chromosomes are the parts your body's cells that carry genes. A human cell usually has 23 pairs of chromosomes.
Genes are single units of genetic material that contain information that you inherit from your parents, such as eye or hair colour. They are carried by chromosomes.

Male and female sex chromosomes

Your sex is determined by chromosomes. You have two sex chromosomes: one from your mother and one from your father.

During early pregnancy, all unborn babies are female because only the female sex chromosome (the X chromosome, which is inherited from the mother) is active. At the eighth week of gestation, the sex chromosome that is inherited from the father becomes active, which can either be an X chromosome (female) or a Y chromosome (male).

If the sex chromosome inherited from the father is X, the unborn baby will continue to develop as female.

If the sex chromosome inherited from the father is Y, the baby will develop as male.

Therefore, a female baby has XX chromosomes and a male baby has XY chromosomes.

Page last reviewed: 13/07/2011

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 that 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 (see box, right).

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:

  • what genes they inherit
  • whether the gene for that condition is dominant or recessive

Gene mutations

Genetic mutations occur when the DNA changes, altering the genetic instructions. This may result in a genetic disorder or a change in characteristics.

Mutations do not just happen 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 it may not be perfect. This can result in a mistake when the DNA is copied while a cell divides, and causes a mutation. 

For example, cigarette smoke is full of chemicals that attack and damage DNA. This leads to mutations in lung cell genes, including ones that control growth. In time this leads to lung cancer.

Mutations can have three different effects:

  • they may be neutral and have no effect
  • they may improve a protein and be beneficial
  • they may result in a protein that does not work, which may cause disease

New mutations

Mutations may be inherited from a parent, or they may occur when a sperm or egg is made, causing 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 on the mutation to their children.

Conditions that are 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 the 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 that are present in the adult. Which one of each pair of chromosomes goes into an egg or sperm is random. So, if a mutation is present in only one of the two copies of a gene (one of a pair of chromosomes), then there is a 1 in 2 chance of it being passed on to a child.

Recessive inheritance

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) and pass it on. 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 1 in 4 chance that their child will have the genetic condition.

For example, a recessive gene causes cystic fibrosis (a condition that causes the internal organs to become clogged with mucus). This means that a child with cystic fibrosis has inherited a faulty gene copy from both their mother and their father.

Dominant inheritance

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 1 in 2 chance that it will be passed on to the child.

A dominant gene causes type 1 neurofibromatosis, which is a condition that causes non-cancerous tumours to grow on nerves throughout the body. A child can inherit type 1 neurofibromatosis if either their mother or father has the condition and passes the faulty gene on. It can also be caused by a new genetic mutation.

X-linked inheritance

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 he will develop the condition. Duchenne muscular dystrophy and haemophilia are inherited in this way.

Multifactorial conditions

Very few conditions are only caused by genes; most are caused by the combination of genes and the environment. The environment includes lifestyle factors, such as diet and exercise.

Around a dozen or more genes determine most human characteristics, such as height and the likelihood of catching common diseases.

It is being discovered that genes may have many variants (alleles). Studies of the whole genome 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 a condition, but in some individuals the risk is significantly above or below the average. This risk may well be reduced by changing environmental and lifestyle factors.

Future research

The way that research is progressing suggests that, in the future, it will be possible for individuals to find out what conditions they are most likely to develop. They can then be advised on how to avoid these conditions by changing their lifestyle and environment.

Lifestyle factors

While your genes may make you more likely to develop a certain condition, the risk of this 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 your 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.

Chromosomes are the parts your body's cells that carry genes. A human cell usually has 23 pairs of chromosomes.
Genes are single units of genetic material that contain information that you inherit from your parents, such as eye or hair colour. They are carried by chromosomes.

Eye colour

The way that you inherit your eye colour is a good example of how you inherit your genes.

The allele (type of gene) for blue eyes is recessive. This means that you need to inherit two blue alleles (one from your mother and one from your father) to have blue eyes.

The allele for brown eyes is dominant. This means that you only need to inherit one brown allele (from either your mother or father) to have brown eyes.

If your mother has blue eyes, she has two blue alleles and you will definitely inherit one blue allele from her. If your father has brown eyes, he could have one brown and one blue allele, or two brown alleles.

If you inherit a brown allele from your father you will have brown eyes. If you inherit a blue allele from your father then you will have blue eyes.

Page last reviewed: 13/07/2011

Genetic testing

Genetic testing is a way of finding out whether you are carrying a particular genetic mutation (altered gene) that causes a medical condition.

Genetic testing usually involves having a sample of your blood or tissue taken. The sample will contain cells that contain your DNA. The sample can then be tested to determine whether you are carrying a particular mutation and are at risk of a particular genetic condition.

Genetic testing is only effective if it is known that a specific genetic mutation causes a condition. For example, it is known that a specific gene causes spinal muscular atrophy (a condition that causes muscle weakness and a progressive loss of movement). It is therefore possible to test a sample of blood 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 only a few particular mutations, so it is relatively easy to test for it in the laboratory. However, Marfan syndrome (a condition that affects the body's connective tissues) can be caused by any one of hundreds of different mutations in a particular gene. 

Gene sequencing

For a condition such as Marfan syndrome, the laboratory has to check all the way through the gene for mutations, using a process called gene sequencing. This has to be done extremely carefully, and can take a long time compared to most other hospital laboratory tests. 

The Marfan's gene, for example, contains more than 200,000 letters of DNA code, divided into 65 parts. This is not even the largest human gene.

Even when a mutation is found, it may be necessary to do a lot of work to determine if that mutation is actually the cause of the condition or not. If, as often happens, a mutation is not found, the diagnosis may have to be reconsidered. There are at least two genes that can cause conditions similar to Marfan's syndrome if they have mutations within them.

Genetics services

The Department of Clinical Genetics at Our Lady’s Children’s Hospital, Crumlin (OLCHC) seeks to provide a comprehensive service for all patients and families in the Republic of Ireland affected by or at risk of a genetic disorder.

For further information in relation to the Genetics service please see www.olchc.ie and www.cuh.ie.

Genetic counselling

Genetic counselling is another term for advice given by healthcare professionals about genetic conditions. It may take the form of a discussion with a healthcare professional who is trained in the science of human genetics (a geneticist). This professional may be:

  • a genetic counsellor
  • a clinical geneticist (a doctor who specialises in genetics)
  • a specialist, such as a paediatrician (a doctor who specialises in childhood health)

The healthcare professional will discuss the risks, benefits and limitations of genetic counselling with you. They will also explain that 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 will usually be very important. Seeing who may have had the condition in the past will help indicate what genetic tests may be appropriate (see above). Geneticists can explain the results of any tests or examinations and help you decide how to progress.

Alternatively, a geneticist may be involved in assessing 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 this with you. They can help you to decide whether you would like to have cancer screening or other further tests.

Chromosomes are the parts your body's cells that carry genes. A human cell usually has 23 pairs of chromosomes.
Genes are single units of genetic material that contain information that you inherit from your parents, such as eye or hair colour. They are carried by chromosomes.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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