What is genetic variation and how does it occur?
April 5, 2023
In our first blog of this series, The Role and Function of DNA, we addressed how individual nitrogen base changes can ultimately alter the proteins that your body produces.
These alterations to the base sequence are examples of genetic variation, a primary driver of the physiological differences that you can see between everyone around you. So, how does genetic variation occur and what is its effect? Let’s get back into the science.
Another way to describe changes to your genetic code is as a ‘mutation’. While this word may carry negative connotations or have you imagining extreme superhero-like consequences, genetic mutations occur all the time, with a mix of negative, neutral and positive effects.
Take lactose tolerance for example. The ability of some people to consume dairy products throughout their life without experiencing digestive issues is itself a result of a genetic mutation. This mutation occurred approximately 8,000 years ago and enables people who carry it to produce lactase, the enzyme that breaks down lactose, beyond infancy. This mutation spread in different populations due to its evolutionary advantage when food was scarce.
Mutations can occur for a variety of reasons, including:
- Errors in DNA replication during cell division
- Exposure to mutagenic factors, such as radiation and chemicals
- Viruses and other pathogens that insert their DNA into that of the host cell
- Spontaneously without any particular cause or reason
SNPs vs. Indel mutations
There are two broad types of mutations:
- Single Nucleotide Polymorphisms (or ‘SNPs’)
- Indel mutations
SNPs are the most common cause of genetic variation between people and were the type described in our The Role and Function of DNA blog. Simply put, SNPs are when a single nucleotide base is swapped for another. An ‘A’ may become a ‘T’, a ‘C’ becomes a ‘G’, and so on.
As you inherit two copies of each gene from your parents, you simultaneously carry two individual variations, known as alleles. The combination of these alleles presents three different versions of each gene, which in turn are known as 'genotypes'. To return to our lactose tolerance example again, the three possible genotypes are:
CC (lactose intolerant)
CT (likely lactose tolerant)
TT (lactose tolerant)
As you can discern from the above example, it’s the mutation of the ‘C’ into a ‘T’ in the DNA sequence near the lactase gene that has allowed certain individuals to digest lactose into adulthood.
Each SNP has a unique identifying ‘rs’ number, which allows it to be specifically located. The rs number for the variant of the lactase gene in your lactose tolerance trait is rs4988235.
The second and less common cause of genetic variation are indel mutations, whereby sections of genetic code are either inserted or deleted (hence the name ‘indel’). This can involve a handful of nucleotide bases, or thousands.
Looking at a health and fitness example, the ‘I’ variant of the ACE gene contains an indel mutation that sees 287 nucleotides inserted to the base sequence. This mutation results in increased activity of the ACE enzyme, which increases blood pressure and promotes fluid retention. Because of these effects, the ACE mutation is associated with various cardiovascular conditions such as hypertension, but seemingly also provides carriers with enhanced endurance performance.
On the other hand, the deletion of nucleotides can reorder the amino acid sequence and fundamentally alter the properties of proteins, even going as far as to cause complete loss of function.
For example, a 2021 paper concluded that approximately 16% of Cystic Fibrosis cases are the result of indel mutations, which result in the production of malformed CFTR protein, or stop production altogether.
The differences between us
Given the amount of variation you see between everyone around you - their height, hair colour, muscle mass, cognitive ability, and even personality - you may assume that we hardly share any of the same genetic information at all.
However, in reality 99.9% of the genetic code between humans is identical. In fact, your DNA sequence is 96% identical to that of a chimpanzee, 80% identical to a cow and 60% identical to a banana.
By analysing the remaining 0.1% that contains your unique genetic variants, we can start to understand how your DNA is influencing your biology. In turn, we can use these insights to identify your optimal health and fitness actions.
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