What you need to know
- 3.20(i)phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. reflects genotypeThe alleles an organism has for a gene. and environmental influences.
- 3.18Protein production links geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. activity to cell processes.
Specific microbes and model methods here are enrichment, not extra specification recall. Example datasets are illustrative.
By the end, you should be able to…
- Distinguish microbiota from the host genome.
- Identify confounding and reverse causationWhen a change in one variable directly causes a change in another. Showing it needs controlled experiments or a known mechanism..
- Design a controlled model investigation.
Build your understanding
The host has neighbours
Microbiota refers to the microorganisms living in a particular environment, such as the gut. Microbiome commonly refers to their collective genetic material and, in broader usage, their communityAll the populations of different species living in one habitat at the same time. and environment. These microbial geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. are not simply extra alleleA different version of the same gene, found at the same locus. in the human nuclear genome.
Microbes can transform substances in their surroundings. Their products may interact with host cells, whose responses depend on their own proteins and regulatory systems. This gives an extension context for genotypeThe alleles an organism has for a gene.–environment interactions; detailed microbial speciesA group of organisms that can interbreed to produce fertile offspring. or pathways are not additional recall for specification 3C.
A fictional association
In a fictional observational dataset, people with a particular diet have both a higher abundance of microbe M and a different value of host measurement P. The pattern does not establish that microbe M causes the difference in P.
Diet might affect both M and P independently. Medication, age or other variables might confound the comparison. The host condition itself might change the microbial habitatThe place where an organism lives., so the direction of causationWhen a change in one variable directly causes a change in another. Showing it needs controlled experiments or a known mechanism. may be reversed. Relative abundance is also a proportion: a rise in M’s percentage need not mean its absolute number increased.
Build a testable model
For an original plant-seedling model, compare genetically matched seedlings given a defined safe microbial preparation with seedlings given the same carrier alone, using a teacher-approved laboratory system. Randomise allocation, replicate independently and control light, water, minerals and age. Measure a predefined outcome such as dry mass.
The control helps isolate the preparation’s effect; it does not prove which microbial molecule acted or that the same outcome occurs in humans. A follow-up comparison with cell-free preparation could test whether soluble products contribute, provided processing and other conditions are controlled. No human sample collection is part of this activity.
Read a claim carefully
Ask what was measured, whether groups were comparable, how large the effect was and how uncertain the estimate is. A plausible mechanism supports a hypothesis; it does not replace evidence. Distinguish a finding in a model system from a broad claim about health.
The central specification link is that phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. is not determined by DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. sequence alone. An environmental influence on gene activity is not automatically a mutationA change in the sequence of bases in DNA. or proof of an inherited epigeneticDescribes changes in gene expression that do not change the DNA base sequence, such as DNA methylation or histone modification. change.
Make your reasoning visible
Name the alternative explanation
“Correlation is not causationWhen a change in one variable directly causes a change in another. Showing it needs controlled experiments or a known mechanism.” is stronger when followed by a plausible confounder or reverse-causation explanation.
Keep organisms distinct
Microbial geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. belong to the microbes; host genes belong to the host. Their interactions do not merge their genomes.
Avoid this misconception
The claim: If a microbe is more common in a group, it must cause that group’s phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment..
The correction: Diet, other variables or the phenotype itself may explain the association.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Microbe M rises from 10% to 20% of a community. Has its absolute abundance necessarily doubled?
Show suggested answer
No. Its proportion can rise if other microbes decline. Absolute counts or suitable quantitative measurements are needed.
Practice question
Retrieval and explanation · suggested answer
Why give control seedlings the carrier solution?
Show suggested answer
It controls for effects of the liquid or other carrier components, so the groups differ more specifically in the microbial preparation.
Practice question
Retrieval and explanation · suggested answer
Give two reasons a microbial association might not show direct causation.
Show suggested answer
A shared factor such as diet could affect both variables; alternatively, changes in the host could alter the microbial community rather than the reverse.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A preparation changes seedling mass only in genotype A, not genotype B. What can you infer, and what remains uncertain?
Show a hint
Separate an interaction pattern from a proven molecular pathway.
Explore the reasoning
The response depends on genotype under the tested conditions, consistent with genotype–environment interaction. The result does not identify the microbial or host mechanism; replication, uncertainty and alternative effects still need evaluation.
Recall this lesson
Lesson recall questions
Question 1 of 5: State what differential gene expression means.
Differential gene expression
Practice question
Retrieval and explanation · suggested answer
State what differential gene expression means.
Lesson: 3C 1: Cell Differentiation
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Microbes as a research field
3C · Thinking Bigger: MicrobiotaNIDDK supports research into interactions between gut microbes, diet, microbial products and host physiology. The research questions connect several biological systems rather than a single “good” or “bad”…Specification 3.20(i)Read & use the scienceClose article
Why this matters
NIDDK supports research into interactions between gut microbes, diet, microbial products and host physiology. The research questions connect several biological systems rather than a single “good” or “bad” microbe. A microbiome study might compare diets, microbial communities and host responses over time. If two groups differ, the researcher still has to ask whether a microbial change caused the phenotype or followed a change in diet or health. Controlled experiments can narrow the possibilities, but human results are rarely explained by one organism acting alone. This links genotype–environment interaction to a concrete research problem. It does not add a new requirement to memorise named gut species for Unit 2.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.20(i) understand how phenotype is the result of an interaction between genotype and the environment
Use the science
Why might changing one microbe have different effects in different hosts?
Explore a possible response
Hosts can differ in genotype, diet, physiology and other community members; these conditions can change the interaction.
Explore the source
- NIDDK: gastrointestinal microbiology researchNational Institute of Diabetes and Digestive and Kidney Diseases
Quantitative genetics
3C · Interactions Between Genes and the EnvironmentNHGRI uses height as an example of a polygenic trait. Researchers studying such traits look beyond a single dominant or recessive allele. Human height is measured on a…Specification 3.21Read & use the scienceClose article
Why this matters
NHGRI uses height as an example of a polygenic trait. Researchers studying such traits look beyond a single dominant or recessive allele. Human height is measured on a continuous scale. Many loci can contribute small effects, and nutrition or illness can also change growth. A researcher studying this trait needs large samples and appropriate controls; a family story alone cannot separate genetic and environmental contributions. An allele linked to greater average height is not a precise prediction for one person. This is the bridge from monohybrid problems to polygenic inheritance: many genotypes produce overlapping phenotypes. The specification asks students to explain variation, not to infer destiny from a single DNA variant.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.21 understand how some phenotypes are affected by multiple alleles for the same gene, or by polygenic inheritance, as well as the environment, and how polygenic inheritance can give rise to phenotypes that show continuous variation
Use the science
Why would one allele usually be insufficient to predict an individual’s exact height?
Explore a possible response
Other loci and environmental influences also contribute, so a single allele does not specify an exact outcome.
Explore the source
- NHGRI: Polygenic TraitNational Human Genome Research Institute
Mapping gene regulation
3C · Controlling Gene ExpressionNHGRI describes epigenomics as the study of chemical marks associated with the genome. Comparing cell types can reveal patterns associated with different gene activity. An epigenetic mark is…Specification 3.20(ii)Read & use the scienceClose article
Why this matters
NHGRI describes epigenomics as the study of chemical marks associated with the genome. Comparing cell types can reveal patterns associated with different gene activity. An epigenetic mark is a chemical modification associated with DNA or histones, not a change to the base sequence. Comparing marks between cell types can suggest how genes are activated or silenced. Researchers also measure mRNA and protein because a mark by itself is not proof of a particular functional outcome. Some marks persist when cells divide, helping maintain specialised states. That does not mean every life experience is inherited by children. The exam chain is mark, altered gene activation, changed mRNA or protein, and possible phenotype.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.20(ii) know how epigenetic modification, including DNA methylation and histone modification, can alter the activation of certain genes
Use the science
Why compare DNA sequence as well as epigenetic marks?
Explore a possible response
A sequence difference could provide another explanation for altered expression; comparing both helps separate the possibilities.
Explore the source
- NHGRI: Epigenomics Fact SheetNational Human Genome Research Institute