IB Biology HLTopic 4 — Genetics, Inheritance & ChangePaper 1 & 2Nature of science~9 min read
Mutations & Genetic Variation
Meiosis and fertilisation shuffle the alleles a population already has. Only one process can produce an allele that has never existed before — mutation. Every version of every gene in every living thing traces back to one, which makes the accidents of copying the most creative force in biology.
📘 What you need to know
Variation means the differences between organisms of the same species — coat colour, body length, flower colour.
Variation comes from small differences in DNA base sequences between individuals.
There are three sources: mutation, meiosis and random fertilisation.
Mutation is the original source of genetic variation, because it is the only one that creates new alleles.
A new allele may be advantageous, disadvantageous or have no apparent effect.
Only mutations in the dividing cells of the sex organs reach gametes and affect the next generation.
Mutation is essential for evolution by natural selection, and is the only source of variation in asexually reproducing species.
Where variation comes from
Two organisms of the same species differ because their base sequences differ slightly. Three processes produce those differences, and it is worth being clear about what each one actually does.
Mutation changes a base sequence, so it makes a version of a gene that did not exist before. This is creating something new.
Meiosis produces gametes with different combinations of the alleles the parent already has, through crossing over and independent assortment. This is shuffling.
Random fertilisation decides which sperm meets which egg, so which combination ends up in the offspring. Also shuffling.
Shuffling a deck endlessly still gives you the same 52 cards. Only mutation prints a new card.
Meiosis and fertilisation work only in sexually reproducing organisms. Mutation works everywhere, which is why it is called the original source.
What happens to a new allele
A mutation in the dividing cells of the sex organs changes the alleles carried by gametes, so it can reach the next generation. From there, its fate depends entirely on whether it helps.
Advantageous — the allele increases the chance that an organism survives and reproduces, so it is more likely to be passed on. Over generations it becomes more common.
No apparent effect — the allele does nothing noticeable. Its frequency drifts up or down more or less by chance.
Disadvantageous — the organism is less likely to survive and reproduce, so the allele is more likely to die out.
Remember the sickle cell allele: harmful on its own, yet an advantage to carriers where malaria is common. “Advantageous” always means “in this environment”.
Somatic mutations do not count here. A mutation in a body cell is never passed on, so it has no effect on natural selection at all — however dramatic its effect on the individual.
The logic examiners want runs in this order: mutation creates a new allele → the allele changes a protein → the protein changes a characteristic → that characteristic affects survival and reproduction → the allele becomes more or less common. Miss out the middle and it reads like magic.
Nature of science: commercial genetic tests
Genetic tests can tell you about your future health and disease risk. There are two very different ways of getting one, and the difference matters.
Feature
Clinical (medical) testing
Commercial testing
Who arranges it
A healthcare provider — doctor, nurse practitioner or genetic counsellor
The customer, by buying a kit online or in a shop
Sample handling
Collected and sent to a laboratory by the provider
Posted to the company by the customer
Who explains the result
The provider, who ensures the patient and family understand the implications
Often nobody — results come straight from the company
Validation
Tests chosen and interpreted by professionals
Some tests are not scientifically validated and can give inaccurate results
The problems with commercial testing are worth learning as a list, because “discuss” questions come up:
Some tests are not scientifically validated, so results may simply be wrong.
Unexpected findings about health, family relationships or ancestry can be stressful or upsetting.
People may make serious treatment decisions based on results they have misunderstood.
Customers are often not offered genetic counselling to help them interpret what they have been told.
Notice that none of these objections is about the science being impossible. They are all about interpretation. That is the nature-of-science point: data without expert context can do more harm than no data at all.
Worked examples
WORKED EXAMPLE
Variation in an asexual species
A species of bacterium reproduces only asexually. State the source of genetic variation in this species and explain why the other sources do not apply.
Step 1: List the three sources
Mutation, meiosis, random fertilisation.
Step 2: Rule out the ones that need sex
Asexual reproduction involves no meiosis and no fertilisation, so neither can shuffle alleles.
Step 3: State what is left
Offspring are otherwise genetically identical to the parent, so any difference must come from a copying error.
Mutation is the only source of variationThis is why new bacterial strains, including resistant ones, still appear.
WORKED EXAMPLE
Explaining a rise in allele frequency
In a population of beetles living on dark bark, a mutation produces a darker shell colour. Over 40 generations the dark allele becomes much more common. Explain why.
Step 1: Start with the allele and the protein
The mutation creates a new allele coding for a different pigment protein, giving a darker shell.
Step 2: Link to survival
Dark beetles are better camouflaged on dark bark, so fewer are eaten by predators.
Step 3: Link to reproduction
They are more likely to survive and reproduce, so they pass the allele to more offspring.
Step 4: Scale it up
Repeated over many generations, the frequency of the dark allele rises.
Natural selection favours the advantageous allele, so it becomes more commonSay “more likely to survive and reproduce” — survival on its own is not enough.
WORKED EXAMPLE
Evaluating a commercial test
A student buys a commercial genetic test kit and is told she has a raised risk of a serious disease. Outline two reasons why she should not act on this result without medical advice.
Reason 1: the result may not be reliable
Some commercial tests are not scientifically validated, so the result may be inaccurate.
Reason 2: the result may be misunderstood
A raised risk is not a diagnosis, and without genetic counselling she may make treatment decisions based on a misreading.
Sensible next step
Take the result to a healthcare provider, who can order a clinical test and explain the implications.
Possible inaccuracy, and no expert interpretationThe word “outline” means two clear reasons, briefly — not an essay.
💡 Exam tip
Say “mutation is the original source of variation” and explain why: it is the only process that produces new alleles.
List all three sources if a question asks about variation in a sexually reproducing species.
For asexual species, remember mutation is the only source.
Describe alleles as advantageous, disadvantageous or having no apparent effect — that is the wording used in the syllabus.
Always add “in that environment” when calling an allele advantageous.
For the genetic testing NOS point, learn two problems with commercial testing and the contrast with clinical testing.
⚠ Common mix-up
Saying meiosis creates new alleles. It creates new combinations of existing alleles. Only mutation makes new ones.
Writing that organisms mutate in order to adapt. Mutations are random and come first; selection acts afterwards.
Counting somatic mutations as a source of variation for evolution. They are never inherited.
Confusing variation with mutation. Variation is the differences you observe; mutation is one of the causes.
Treating a genetic risk as a diagnosis. A raised risk is a probability, not a certainty.
Saying natural selection changes an individual. It changes allele frequencies in a population over generations.
Up next: Gene Editing — if mutation is random and cells cannot change their own DNA on purpose, what happens when we do it for them? Time for knockout organisms and CRISPR.
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