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Genetics - How genetic conditions are inherited

NHS Choices Medical Reference

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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

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

New mutations

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.

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) 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.

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 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.

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

Multi-factorial conditions

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.

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. 

Medical Review: July 28, 2012
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