Key term
AQA GCSE Combined Science Biology 8464
Bioenergetics
Cell Biology
Topic overviewEukaryotes and prokaryotesAnimal and plant cellsCell specialisation and differentiationMicroscopyChromosomes, mitosis and the cell cycleStem cells in embryos, adults and meristemsTherapeutic cloning, its risks, and cloning plantsDiffusion and the factors affecting its rateSurface area to volume ratio and exchange surfacesOsmosisActive transport
Ecology
Topic overviewLevels of organisation in an ecosystem, and what organisms needCompetition, interdependence and stable communitiesAbiotic factorsBiotic factorsAdaptationsProducers, food chains and sampling with quadrats and transectsPredators, prey and measuring population sizeHow materials are cycledWaste management and biodiversityLand use, deforestation and global warmingMaintaining biodiversity
Homeostasis and Response
Infection and Response
Inheritance, Variation and Evolution
Topic overviewSexual and asexual reproductionMeiosisDNA and the genomeGenetic terms, dominant and recessive allelesPunnett squares, probability and genetic crossesInherited disorders and sex determinationVariationEvolutionSelective breedingWhat genetic engineering is, and what it is used forGM crops, their risks, and the steps of the processEvidence for evolution, and extinctionFossilsResistant bacteriaClassification of living organisms
Organisation
Topic overviewLevels of organisation, and how enzymes workAmylase, proteases and lipasesBile, food tests and the effect of pH on amylaseThe heart, the lungs and double circulationThe pacemaker, the blood vessels and the bloodCoronary heart disease: a non-communicable diseaseHealth issuesRisk factors and causal mechanisms in diseaseSampling, correlation and interpreting risk dataCancerPlant tissues, and transpiration and its rateRoot hair cells, xylem, phloem and stomata
Inheritance, Variation and Evolution
Punnett squares, probability and genetic crosses
- 3 things to know
- Biology Paper 2
Overview
Characteristics controlled by several genes, why a cross predicts only probabilities, ratios and percentages from 3:1 and 1:1 crosses, and worked crosses in mice, cats, peas and a recessive disorder.
What you need to know
Answer each one before you reveal it.
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Key term
Why can a genetic cross only predict the likely proportions of the offspring?
Key term
In a family tree, a characteristic appears only in children whose parents do not show it. Is the allele responsible dominant or recessive?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Are most characteristics controlled by one gene or by many?
Model answer
Most are the result of several genes interacting rather than a single gene.
- 2
A characteristic shows a wide range of values across a population rather than two clear types. Suggest why.
Model answer
It is controlled by several genes interacting, not by a single gene.
- 3
In a single gene cross the chance of an offspring showing a phenotype is 1 in 4. If 100 offspring are produced, will exactly 25 show it?
Model answer
Not necessarily, because the chance applies separately to each offspring, so the actual number varies around 25.
- 4
Why can a genetic cross only predict the likely proportions of the offspring?
Model answer
Which allele each gamete carries is a matter of chance, so the result is a probability rather than a certainty.
- 5
A cross produces offspring in the ratio 3 round seeds to 1 wrinkled seed. What percentage of the offspring have wrinkled seeds?
Model answer
25%.
- 6
A cross gives 3 tall plants for every 1 short plant. Out of 60 offspring, how many are expected to be tall?
Model answer
45 plants.
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