What is Genetic Variation?
The differences in DNA sequences between individuals of the same species, resulting in different traits and characteristics.
Look around your classroom — even identical twins have subtle differences. Now think about the enormous range of differences between all the humans on Earth: different heights, eye colours, blood types, disease resistances, and thousands of other traits. This diversity is the result of genetic variation.
Genetic variation is not just interesting — it is essential for the survival of species. Without it, a single disease or environmental change could wipe out an entire population. It is the raw material that natural selection acts upon, driving evolution over generations.
Think of a species' gene pool like a recipe book. Genetic variation means the recipe book has many different versions of each recipe. If one version fails (a harmful mutation), there are other versions to fall back on. A species with no variation has only one recipe — if that recipe goes wrong, there is nothing to save it.
In MYP Sciences, we focus on three main sources of genetic variation:
- Mutations — changes to DNA sequences
- Meiosis — the special cell division that produces gametes
- Fertilisation — the random fusion of gametes
In the MYP Sciences framework, this topic connects directly to the key concept of Change and the related concept of Variation — understanding how and why individuals within a species differ is central to explaining how life adapts and evolves over time.
Mutations: Changes to the DNA Code
A permanent change in the DNA nucleotide sequence of an organism's genome.
Mutations are the ultimate source of all genetic variation. Every new allele (version of a gene) that exists today arose at some point as a mutation. Mutations can occur in two main ways:
- Spontaneous mutations — random errors that occur during DNA replication when cells divide
- Induced mutations — caused by external agents called mutagens
Common mutagens include:
- Ultraviolet (UV) radiation from the sun
- X-rays and gamma rays (ionising radiation)
- Certain chemicals (e.g., benzene, some pesticides)
- Some viruses
Types of gene mutations:
- Substitution — one base is swapped for another (e.g., A→G)
- Insertion — an extra base is added
- Deletion — a base is removed
Insertions and deletions of a single nucleotide (or any number that is not a multiple of three) are called frameshift mutations because they shift the reading frame of the entire DNA sequence after the mutation point, usually changing many amino acids downstream and often producing a non-functional protein. However, if the number of inserted or deleted bases is a multiple of three, the reading frame is preserved — this is called an in-frame mutation and tends to have a milder effect.
Gene mutations vs. chromosomal mutations:
So far we have described mutations affecting individual bases — called gene mutations. Mutations can also occur at a larger scale, affecting entire chromosomes. These are chromosomal mutations.
One important type is non-disjunction — the failure of homologous chromosomes (or sister chromatids) to separate properly during meiosis. This produces gametes with the wrong number of chromosomes. If such a gamete is fertilised, the resulting zygote will have an abnormal chromosome number.
For example, if non-disjunction affects chromosome 21 during meiosis, a gamete may carry two copies of chromosome 21 instead of one. After fertilisation with a normal gamete, the zygote has three copies of chromosome 21 — a condition called trisomy 21, which causes Down syndrome.
Are mutations always harmful?
- Most mutations are neutral — they have no effect on the organism
- Some are harmful — they disrupt protein function and may cause disease
- A small number are beneficial — they improve an organism's ability to survive and reproduce
A common misconception is that mutations are always dangerous. In reality, the vast majority of mutations have little or no effect because they occur in non-coding DNA, or because the genetic code allows multiple codons to code for the same amino acid — meaning some base changes do not alter the protein produced at all.
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