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.
