Genes and thinking skills
Genes provide the blueprint for life. They’re the code written into our DNA. They influence many things about us, including thinking skills as well as physical traits like eye and hair colour.
What are genes?
Our bodies are made up of trillions of cells, each with a nucleus. The nucleus houses chromosomes, which are made up of DNA. In turn, DNA contains sequences of genetic code. Each sequence is known as a ‘gene’. Genes provide vital instructions to cells to make proteins that carry out particular functions in the body, e.g. building or repairing bone or muscle cells.
We inherit our genes from our parents. This is the way characteristics and conditions are passed down the generations in families. We have two copies of each gene, one inherited from each parent.
Most genes are the same in everyone. A small number (about 0.1% of the total) are different between people. This explains the differences between us that make each of us unique.
Do genes influence thinking skills?
As individuals, we vary widely in the level of our thinking skills, or ‘cognitive function’.
We inherit cognitive function from our parents, in the same way that physical characteristics are passed down. Scientists have discovered that, unlike eye colour, cognitive function is not influenced by a few genes but by many. Individually, each gene has a small influence but together they account for more than half (or 50%) of the differences in cognitive function between us.
In addition, The Disconnected Mind research project team at the University of Edinburgh has shown that the majority of genetic effects that act on cognitive ability in old age are those that act on cognitive ability in childhood. This means that people with a higher level of cognitive ability in youth are more likely to have a higher level of ability throughout their lives and in older age.
Scientists are still working to identify genes which contribute to our capacity to think and learn, as so far only a small number have been identified. One gene that has reliably been associated with cognitive function, particularly in older people, is APOE. This gene is involved in cholesterol transport and is likely to influence brain function.
Genes and ageing of thinking skills
It’s normal for some of our thinking skills to decline slightly as we grow older – including speed of thinking, some types of memory and manipulating new information – while others stay stable or increase, for example wisdom and vocabulary.
But either side of the ‘norm’, we vary widely in the extent and rate of change. In other words, some people may do better in their thinking skills than the norm and others may decline more or faster.
The extent and rate of decline as we age is affected by both genetic influences (what we were born with) and ‘environmental’ influences, i.e. what we encounter or are exposed to during our lives. This includes life experiences such as education, health, lifestyle choices, attitude and emotions, and a range of other factors.
The Disconnected Mind project team studies the relative contribution of genetic and environmental influences to change in cognitive function as we age.
They discovered that approximately 24% of the extent of change between childhood and older age (age 70) is due to genetic influences. This means that environmental influences make a larger contribution, at approximately 76%.
Where these are things we can control or alter (e.g. the choice to smoke or not; decisions we make about diet), we can make changes to help protect our cognitive health as we age.
How do environmental factors affect genes?
The genes we are born with never change but they don’t all need to produce the proteins they are responsible for all the time. This means that they can be switched ‘on’ and ‘off’ during our lives. This is known as gene regulation.
Much of the switching that takes place is natural – a normal part of our everyday biology – and is vital over the life course, for instance to build and repair different body cells at the right times. Switching can also happen in response to environmental factors and disease. This enables cells to react quickly to changes in their environment.
Researchers are increasingly interested in studying what switches genes on and off. This relatively new branch of biomedical science is known as ‘epigenetics’. We don’t fully understand it yet, but we do know that the switching is done by chemical modifications to the DNA.
One of the types of modification that has been identified so far is known as ‘methylation’, a process in which methyl groups are added to genes. This changes the activity of that gene without changing the gene itself. In other words, DNA methylation helps control how active or inactive our genes are.
Epigenetics, ageing and the future
The Disconnected Mind project team is studying what environmental factors trigger DNA methylation of genes that are associated with ageing. They have found, for example, that smoking is an environmental influence that has a large effect – it’s possible to spot current smokers by looking at methylation levels in a single gene.
Links between DNA methylation and schizophrenia and Alzheimer’s disease have also emerged, but further research is needed to find out whether this offers potential for prevention or treatment of these conditions.
The study of epigenetics and epigenetic differences between people is also providing findings on the rate of ageing and how long we live, information about the mechanisms behind diseases and the biological implications of our lifestyle choices.
In time, it will help us learn more about ageing of our brains and thinking skills, providing more understanding of how we can protect their health as we grow older.
- Relton, C.L., Hartwig, F.P., Davey Smith, G., 2015. From stem cells to the law courts: DNA methylation, the forensic epigenome and the possibility of a biosocial archive. Int J Epidemiol, 44:4, pp.1083-93.
With thanks for this page to scientists in the University of Edinburgh’s Centre for Cognitive Ageing and Cognitive Epidemiology.