🗒️ What This Blog Post Is All About?
The terms genetic mutation and genetic variant keep popping up throughout my blog posts. I did not do statistics, but I would assume that somatic and germline mutations/variants are probably among the most common terms in my blog posts.
Besides the above-written, I also realized that many of you do not have much background in genetics and potentially don’t know what the terms germline and somatic mutations/variants mean and what the difference is between these two.
So, I decided to write this blog post where I will try to explain these two terms, explain additional related terms and processes, and also explain the critical differences between somatic and germline mutations/variants.
🧲What Is the Difference Between Genetic Mutation and Genetic Variant?
Before I explain what germline and somatic mutation are and what is the difference between the two, I will first explain what is a genetic mutation and genetic variant and explain the difference between these two.
One might assume that there is a difference between the terms genetic mutation and genetic variant. Still, I will consider them synonymous here for simplicity and explain why I consider them synonymous.
To my knowledge, there is a very subtle difference between genetic mutation and genetic variant.
⚠️ The term genetic mutation, in common parlance, has been used with a negative connotation, referring to changes in the DNA sequence that have adverse effects (basically causing diseases). However, this is not the case, as the term mutations refer to any genetic change that is harmful but also beneficial or neutral in its effect on cell function.
⚠️On the other side, the term genetic variant has been used, again in common parlance, to describe any change in DNA sequence independently of their effect.
However, these terms can, in my opinion, be considered synonymous. So, from now on in this blog post, I will exclusively use the term genetic mutation and, as such, be considered synonymous with the term genetic variant.
The main reason for using the term genetic mutation in this blog post as a main term is the fact that the term genetic mutation is more often used in common parlance than a genetic variant.
✖️ What Is a Genetic Mutation?
So, now that we settled the story about genetic mutation and genetic variants let’s finally explain what a genetic mutation is. By genetic mutation, we consider any change or alteration in one’s DNA sequence. I assume you know what DNA is?
🧬 What is DNA?
⚠️ If not, let’s just briefly introduce DNA. DNA, or deoxyribonucleic acid, is the molecule that carries the instructions for making all the proteins and some other molecules in our bodies. These instructions are encoded in the DNA sequence by using four nucleotide bases:
adenine (A)
guanine (G)
cytosine (C), and
thymine (T)
⚠️ DNA is packed in chromosomes, and we have two sets of chromosomes each set getting from one of our parents.
✖️ What Is a Genetic Mutation Cont.?
Now, let’s go back to genetic mutations. That DNA sequence we obtain from our parents can change, alter, mutate and simply illustrate if it was:
…..CTGCGTGAGAGAGCGA…..
it can become:
…..CTGCGTGCGAGAGCGA…..
So, in this dummy example, A is replaced with C in this short segment of DNA. This is just one example, as there are other types of mutations, but you can read about that in the following paragraphs.
🤹 How Can Genetic Mutations Be Classified?
There are multiple ways to classify genetic mutations, and the specific classification used will depend on the context. Just for illustration, I will provide several examples of how genetic mutations can be classified:
Genetic mutations can be classified by the type of change that occurs in the DNA sequence, meaning the structural consequence of the genetic mutation. Following this criteria, genetic mutations can be so-called point mutations (single nucleotide polymorphism) corresponding to changes in a single nucleotide base (like the example I explained above). On the other side, deletions involve removing one or more nucleotides from the DNA sequence. Also, insertions correspond to the insertion of one or more nucleotides into the DNA sequence.
Genetic mutations can also be classified based on their effect on the protein encoded by the given gene. For example, a missense mutation is a change in a single nucleotide base that results in the production of a protein with a different amino acid. In contrast, a nonsense mutation is a change that leads to the production of a shortened, nonfunctional protein.
Genetic mutations can also be classified by inheritance pattern. This means that genetic mutations can be classified based on how they are inherited from a person's parents. For example, a dominant mutation is expressed even if only one copy of the mutated gene is present. In contrast, a recessive mutation is only expressed when two copies of the mutated gene are present.
Genetic mutations can also be classified by penetrance, meaning that different genetic mutations have different likelihoods leading to the development of the associated disorder or trait. A mutation can be classified as having high penetrance if it almost always leads to developing the disorder or trait. In contrast, a mutation with low penetrance may not always result in the disorder or trait.
Finally, mutation can also be classified by what types of cells they occur, and this is the classification that I will cover in the blog post. By this classification, mutation can be grouped into germline/germinal/gametic mutations that occur in germinal cells, which are sperm and egg cells, and somatic that occur in all other types of cells, but not germline cells.
📊 The Most Common Classification of Genetic Mutations
However, the most common classification of genetic mutations/variants is one that groups genetic variants into the following three groups:
Single base-pair substitution or also called single-nucleotide polymorphisms, cover the substitution of a single nucleotide of any type.
Insertion or deletion, also known as ‘indel’ is a type of genetic variant that is characterized by the insertion or deletion of a single stretch of DNA sequence that can range from two to hundreds of base-pairs in length
Structural variation is a genetic variant that is characterized by the fact that occurs over a more extensive DNA sequence and includes both copy number variation and chromosomal rearrangement events.
🧬 What Is Somatic Mutation?
⚠️ Somatic mutation is a mutation occurring in a somatic cell. Somatic cells are all cells in the human body except sperm and egg cells, and sperm and egg cells are called germ cells.
Following the analogy, germline mutations of mutations that occur in germ cells (sperm and eggs).
So, somatic mutations are spontaneously occurring and accumulating in somatic cells throughout one's life.
Most of these somatic mutations do not have an observable effect. However, some can disrupt critical cellular functions leading to cancer and contributing to aging.
Somatic mutations that occur early in life can cause developmental disorders, while the continuous accumulation of somatic mutations throughout one’s life can lead to cancer and contribute to aging.
✍️ Examples of Somatic Mutations
If you want to check some concrete examples of somatic mutations that lead to onset of cancer check some of the following blog posts:
🧬 What Is Germline Mutation?
⚠️ On the other hand, as I already mentioned, germline mutations are mutations that occur in germ cells (sperm and eggs).
🧲 What Is the Difference Between Somatic and Germline/Gametic/Germinal Mutations?
⚠️ Since sperm and egg cells are from which we get DNA from our parents, germline mutations are inherited from parents to offspring, and somatic mutations are not inherited.
⚠️ Therefore, somatic mutations affect only their descendants and will not be heritable since it is not present in the germ cells and germline mutations are passed from parents to offspring.