Genetic inheritanceGenetics
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.
For example, for the gene that determines eye colour, you may inherit a brown-eye gene from your mother and a blue-eye gene from your father.
In this instance, you'll end up with brown eyes because brown is the dominant gene. The different forms of genes for eye colour are caused by changes (mutations) 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
- 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 3 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 on to a child by his or her parents.
Depending on the specific condition concerned, these genetic conditions can be inherited in 3 main ways, outlined below.
Autosomal recessive inheritance
Some conditions can only be inherited in an autosomal recessive pattern. This means the condition can only be passed on to a child if both parents have a copy of the faulty gene – both are "carriers" of the condition.
If the child only inherits 1 copy of the faulty gene, they'll be a carrier of the condition but won't have the condition.
If a mother and a father both carry the faulty gene, there's a 1 in 4 (25%) chance of each child they have inheriting the genetic condition, and a 1 in 2 chance (50%) of their child 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
Some conditions are inherited in an autosomal dominant pattern. In this case, only 1 parent needs to carry the mutation for the condition to be passed on to the child.
If 1 parent has the mutation, there's a 1 in 2 (50%) chance it will be passed on to each child the couple has.
Examples of genetic conditions inherited in this way include:
- type 1 neurofibromatosis – a condition that causes tumours to grow along the nerves
- tuberous sclerosis – a condition that causes mainly non-cancerous (benign) tumours to develop in different parts of the body
- Huntington's disease – a condition where certain brain cells become increasingly damaged over time
- autosomal dominant polycystic kidney disease (ADPKD) – a condition that causes small, fluid-filled sacs called cysts to develop in the kidneys
X-linked inheritance
Some conditions are caused by a mutation on the X chromosome (1 of the sex chromosomes). These are usually inherited in a recessive pattern, but in a slightly different way from the autosomal recessive pattern described above.
X-linked recessive conditions often do not affect females to a significant degree because they have 2 X chromosomes, 1 of which will almost certainly be normal and can usually compensate for the mutated chromosome. However, females who inherit the mutation will become carriers.
Males cannot inherit X-linked mutations from their fathers because they receive a Y chromosome from them. A male will develop the condition if he inherits the mutation from his mother. This is because he does not have the normal X chromosome to compensate.
When a mother is a carrier of an X-linked mutation, each daughter they have has a 1 in 2 (50%) chance of becoming a carrier, and each son they have has a 1 in 2 (50%) chance of inheriting the condition.
When a father has an X-linked condition, his sons will not be affected because he'll 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 – caused by having an extra copy of chromosome 21
- Edwards' syndrome – caused by having an extra copy of chromosome 18
- Patau's syndrome – caused by having an extra copy of chromosome 13
- Turner syndrome – a disorder that only affects females and is caused by a missing or abnormal X chromosome
- Klinefelter's syndrome – a disorder that only affects males and is caused by an extra X chromosome
While these are genetic conditions, they're generally not inherited. They usually occur randomly as a result of a problem before, during or soon after the fertilisation of an egg by a sperm.
Influence of the environment
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 people are showing 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's thought 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, individuals will be able to find out what conditions they're most likely to develop.
It may then be possible to significantly reduce the chances of developing these conditions by making appropriate lifestyle and environmental changes.