Experts do not know what causes Parkinson’s disease, but genetic and environmental factors appear to play a role.

Although some cases of Parkinson’s disease run in families, most people with a diagnosis do not have a family history of the condition.

Worldwide, around 10–15% of people with a diagnosis of Parkinson’s disease have a family history of it. In 85% of cases, there is no family history.

Parkinson’s disease affects movement, balance, and other functions. Some people also experience emotional changes and dementia.

Symptoms usually start after the age of 60 years. Late onset Parkinson’s disease appears after the age of 50 years. If symptoms appear before the age of 50 years, it is early onset Parkinson’s disease. Less commonly, it can occur before the age of 20 years. Doctors call this juvenile Parkinson’s disease.

People who have three or more relatives with Parkinson’s disease, especially if they develop symptoms before the age of 50 years, are more likely to have a genetic cause for Parkinson’s disease. Most people with a diagnosis of the condition do not know of any other family members with it.

In this article, learn about how genetic factors affect the development of Parkinson’s disease and whether or not to seek genetic testing.

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Photo editing by Stephen Kelly; Photography by Peter Dazeley/Getty Images

Parkinson’s disease can be hereditary, but most cases are not. Only around 15% of people with Parkinson’s disease have a family history of the condition.

As well as genetic changes, environmental factors — such as exposure to toxins, the use of certain drugs, and traumatic brain injury — may also increase the risk. Most cases develop without a clear reason.

If a sibling or family member has Parkinson’s disease, an individual may have a higher chance of developing it themselves. However, most cases are not hereditary.

In addition, not everyone with the relevant genetic features or family history will develop Parkinson’s disease. A person might have features that increase the risk but never develop symptoms.

Most likely, environmental factors also need to be present.

The symptoms of Parkinson’s disease appear to occur when the brain is no longer able to produce enough dopamine. Low dopamine levels in the brain can affect movement. It is not yet clear what role genetic factors may play in this process.

However, experts have identified specific genes in which changes appear to increase the risk of developing Parkinson’s disease. The symptoms a person experiences may depend on their specific genetic changes.

Genetic changes can affect how mitochondria work. Mitochondria are the parts of a cell that produce energy. As they do this, they release byproducts commonly known as free radicals. Free radicals can cause cell damage.

Usually, cells can counter free radicals, but genetic changes can stop this from happening, and the free radicals can cause damage to dopamine cells.

Genetic changes can also lead to accumulations of a protein called alpha-synuclein in and around neurons throughout the brain. These accumulations are known as Lewy bodies, and the damage they cause can result in Lewy body dementia, which has links to Parkinson’s disease.

Dopamine-producing nerve cells appear to be particularly susceptible to Lewy bodies, and some people develop both Parkinson’s disease and Lewy body dementia.

Symptoms of Lewy body dementia include REM sleep behavior disorder, changes in thinking, hallucinations, and Parkinsonism. Parkinsonism refers to any condition that involves tremors, slowness of movement, or other symptoms similar to those that occur with Parkinson’s disease.

Specific gene changes have specific outcomes. For example, SNCA affects the processing of alpha-synuclein, and PRKN impacts how mitochondria work.

The genetic changes involved in Parkinson’s disease can be:

  • autosomal dominant
  • autosomal recessive
  • epigenetic

Studies have found different genes linked to Parkinson’s disease in different patterns of inheritance. They are still investigating whether or not these genes can follow both patterns and if they are more likely to be present in people who develop symptoms before the age of 30 years.

Autosomal dominant genetic features

People have two copies of each gene. In autosomal dominant inheritance, a child can inherit either a healthy gene or one that is not working correctly. They will have a 50% chance of inheriting a faulty gene.

Autosomal dominant genes that scientists have associated with Parkinson’s disease include:

  • SNCA, or PARK1
  • UCHL1, or PARK5
  • LRRK2, or PARK8
  • PARK3

Autosomal recessive genetic features

In autosomal recessive inheritance, two parents have a faulty gene but not the condition linked to it. This is because they still both have one healthy gene.

However, their offspring can inherit two copies of the faulty gene. The offspring will have a 25% chance of inheriting two faulty genes.

Inheriting two faulty copies of the following genes, one from each parent, increases the risk of Parkinson’s disease:

  • PARK2
  • PARK7
  • PINK1, or PARK6
  • DJ-1
  • PRKN

It is important to note that inheriting any of the genes that scientists have identified as being related to Parkinson’s disease does not necessarily mean that a person will develop the condition.

Changes that are not hereditary

Some genetic changes can result from environmental factors. A person is not born with these; they develop during their life.

This type of change, known as epigenetic, may affect their risk of Parkinson’s.

Research into genetic links with Parkinson’s disease is ongoing.

Genetic testing is available for some genes related to Parkinson’s disease, but testing may not provide useful information to individuals.

For one thing, a wide range of genes may play a role, and it is not possible to test them all. A person may also have a relevant feature but not go on to develop Parkinson’s disease.

For example, only around 0.7% of people with symptoms of Parkinson’s disease have changes in the LRRK2 gene, and around 0.3% have changes in the PRKN gene, according to a 2020 review.

Finding out in advance if a young person has the gene may help them prepare for the future if there is strong evidence of a family history of the condition. However, the results are unlikely to be conclusive and may cause unnecessary anxiety.

Anyone who is interested in genetic testing should discuss the pros and cons with a doctor and consider genetic counseling if they decide to go ahead.

The Parkinson’s Foundation notes that testing is often hard to access. It can also be costly, and health insurance may not cover it. Genetic counseling can be an additional cost.

As well as family history, the following factors may increase the risk of Parkinson’s disease:

  • being aged 60 years or above
  • being male, as Parkinson’s disease affects around 50% more males than females
  • having past exposure to certain toxins, such as pesticides
  • having a history of head trauma or concussion
  • having certain medical conditions, such as an infection

Learn more about the risk factors for Parkinson’s disease here.

Parkinson’s disease involves a range of symptoms, and it can affect people differently.

The four key symptoms that doctors usually look for during diagnosis are:

  • tremors
  • muscle stiffness or rigidity
  • slow movement, or bradykinesia
  • changes in posture and balance that can increase the risk of falls

Symptoms may start in one side of the body but move to other parts in time. They can range from mild to severe. Some people will only ever develop mild symptoms, while others may become unable to take care of themselves.

Learn more about the stages and symptoms of Parkinson’s disease here.

It is not possible to prevent Parkinson’s disease, but some actions that may reduce the risk of developing it include:

  • avoiding exposure to toxins and using protective gear when exposure is unavoidable
  • wearing protective headgear during contact sports and other activities where there is a risk of head trauma
  • monitoring for side effects such as tremors when using medications
  • exercising, which may help reduce the risk or slow progression if it does develop

Parkinson’s disease has a genetic component, but most cases do not run in families. Although a person may inherit genetic changes that can lead to symptoms, this is not guaranteed to happen.

Other factors, such as toxin exposure and head injuries, also appear to play a role. Often, however, Parkinson’s disease has no clear cause.