Genetics, Populations, Evolution and Ecosystems (A Level only)

7. Genetics, Populations, Evolution and Ecosystems (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

15 min read 24 parts Lesson 1 of 25 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision

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Lesson summary

Genetics, Populations, Evolution and Ecosystems — notes: Predict inheritance with monohybrid, dihybrid, linked and epistatic crosses and chi-squared tests, apply the Hardy-Weinberg principle, explain selection, speciation and drift, and investigate populations, succession and co…

Genotype and phenotype
Codominance
Monohybrid cross
Codominant example
Sex linkage
Probability example

Part 1 of 22Genetic key terms and dominance

Genetic key terms and dominance

Gene

Gene a DNA sequence coding for a polypeptide at a specific locus;
- alleles are versions of the gene with slightly different base sequences at the same locus.

Genotype and phenotype

Genotype and phenotype genotype is the alleles present;
- phenotype is the observable characteristic, from genotype and environment.
Homozygous
- two identical alleles;
  - heterozygous: two different alleles.

Dominant

Dominant allele is always expressed (in homozygotes and heterozygotes); recessive allele is expressed only in homozygotes.

Horse coatAA and Aa black, aa chestnut.

Codominance

Codominance both alleles in a heterozygote are fully expressed.
Blood groupsgene I with alleles I^A, I^B (codominant) and I^O (recessive).
I^AI^A or I^AI^O = A; I^BI^B or I^BI^O = B; I^AI^B = AB; I^OI^O = O.

F1 and F2

F1 and F2 homozygous dominant × homozygous recessive gives all heterozygous F1;
- F1 × F1 gives F2 with 3 dominant : 1 recessive and 1 AA : 2 Aa : 1 aa.

Notation

Notation capital for dominant and lower case for recessive using letters that look different (B/b, not C/c).

Part 2 of 22Monohybrid crosses and sex linkage

Monohybrid cross

Monohybrid cross one gene; each allele has an equal chance of entering a gamete; predictions are probabilities,
- and real ratios vary because fertilisation is random.
Bb × Bb: 3 black : 1 chestnut; 1 BB : 2 Bb : 1 bb.
Always show parental phenotypes, genotypes, gametes, then the square, then offspring genotypes and phenotypes.

Codominant example

I^AI^O × I^BI^O gives 1 AB : 1 A : 1 B : 1 O.

Sex linkage

Sex linkage genes on the X chromosome (rarely Y); females XX, males XY.
Males have one copy, so show recessive traits more often and cannot be carriers.
Males do not pass X-linked traits to sons.
Haemophilia (Factor VIII gene)X^HX^h × X^HY gives 1 normal female : 1 carrier female : 1 normal male : 1 haemophiliac male.

Probability example

Sarah's father had haemophilia, so she is X^FX^f;
- Mike is X^FY;
- the chance of an affected baby is 1 in 4, or 25%.
Read whether the question asks about all children or one sex.

Part 3 of 22Dihybrid crosses, linkage and epistasis

Dihybrid cross

Dihybrid cross two genes at once; write both alleles of one gene first (BbEe, not BEbe).
BbEe × bbEe (black brown-eyed × chestnut brown-eyed): 3 black brown : 3 chestnut brown : 1 black blue : 1 chestnut blue.
Heterozygous × heterozygous gives 9 : 3 : 3 : 1 (56.25%, 18.75%, 18.75%, 6.25%).

Autosomal linkage

Autosomal linkage genes on the same chromosome do not assort independently and stay together, written in brackets, for example (TG)(tg).
(TG)(tg) × (tg)(tg) gives 1 normal tail, green : 1 short tail, white.
Ratios that depart from 9:3:3:1 suggest linkage; crossing over produces some recombinants.

Epistasis

Epistasis one gene affects the expression of another on a different chromosome.
Pigeons
- R gives grey feathers (r gives white), but dominant F blocks grey;
  - only R_ff is grey and all other genotypes are white.

Part 4 of 22Test crosses and planning genetic diagrams

Test cross

Test cross cross an individual with the dominant phenotype with a homozygous recessive to find its genotype.
Monohybrid
- no recessive offspring means homozygous dominant;
  - any recessive offspring means heterozygous.
Dihybrid
- no recessive for either gene means homozygous dominant for both;
  - recessive for one gene only means heterozygous for one;
  - recessives for both means heterozygous for both.
Rabbit Floppy (long ears D)a short-eared (dd) offspring shows Floppy is Dd.

Before drawing

Before drawing how many genes, how many alleles, which is dominant, the type of dominance, and any linkage or epistasis.
- (complete, codominance, incomplete)

Part 5 of 22Investigating genetic ratios

Model organisms

Fast Plants (Brassica rapa) and Drosophila have short life cycles and single-gene traits.

Fast Plant stem colour

A dominant to a (green).
- (anthocyanin, purple)
AA × aa: all Aa, purple.
Aa × Aa: 1 AA : 2 Aa : 1 aa, so 3 purple : 1 green.

Drosophila wing length

L (wild type) dominant to l (vestigial).
Ll × Ll3 wild type : 1 vestigial (75% : 25%).

Reminder

Reminder an individual showing the recessive phenotype is always homozygous recessive.

Part 6 of 22The chi-squared test

Find the observed (O) and expected (E) values for categories of data.

Calculate O – E.

Square each difference.

Divide each by E.

Add the results: χ² = Σ (O – E)² ÷ E.

Degrees of freedom = number of classes – 1.

Compare with the critical value at p = 0.05.

Decision

χ² greater than or equal to the critical value: significant difference, reject the null hypothesis.
χ² smaller: no significant difference, differences due to chance.

Worked example

128 rabbits, expected 9:3:3:1 (72, 24, 24, 8).
χ² = 0.014 + 0.375 + 0.167 + 0 = 0.56.
Degrees of freedom = 3; critical value at 0.05 is 7.82.
0.56 is smaller, so no significant difference; probability above 10% that it is chance.

Significant difference

Significant difference suggest an unaccounted factor such as linkage.

Part 7 of 22Species, populations and gene pools

Species, populations and gene pools

Species

Species a group of similar organisms that can reproduce to give fertile offspring and share the same chromosome number.
Horse (64) × donkey (62) gives a mule with 63 chromosomes that cannot pair in meiosis, so it is infertile.

Population

Population organisms of one species in a particular space at a particular time that can potentially interbreed.

Gene pool

Gene pool all the alleles of the genes in a population at a time;
- allele frequency is how often an allele occurs.
Allele frequencies change with natural selection, and large changes lead to evolution or new species.

Phenotype frequency

Phenotype frequency individuals with the phenotype ÷ total individuals × 100.

Of 9 pea plants, 7 purple (78%) and 2 white (22%).

Part 8 of 22Hardy-Weinberg principle

Principle

Principle allele frequencies stay constant between generations if the population is diploid, reproduces sexually only, has non-overlapping generations, mates randomly, is large,
- and has no migration, mutation or selection.
- (rarely all true in nature)

Equations

Equations for alleles B (frequency p) and b (frequency q)

p + q = 1.
What the symbols mean: p = frequency of the dominant allele · q = frequency of the recessive allele · p² = homozygous dominant · 2pq = heterozygous · q² = homozygous recessive
p² + 2pq + q² = 1 (BB, Bb, bb).
Frequencies are between 0 and 1.

Worked example

10% of birds are white (ff), so q² = 0.10.
q = 0.32 and p = 1 – 0.32 = 0.68.
FF = p² = 0.46; Ff = 2pq = 0.44; ff = 0.10; total = 1.

Tip

Tip start with q², the recessive phenotype frequency; the equations estimate frequencies while the principle assumes no change.
- (the only genotype known directly)

Part 9 of 22Sources of genetic variation

Sources of genetic variation

Phenotypic variation

Phenotypic variation genetic variation + environmental influence.

Genetic
- ABO blood groups;
  - environmental: plant height;
  - both: sickle cell allele maintained where heterozygotes resist malaria.

Sources of genetic variation

Mutationnew alleles, and it must occur in gametes to be inherited.
Crossing over of non-sister chromatids in prophase I: new allele combinations on chromosomes.
Independent assortment (segregation) in metaphase I: different chromosome combinations in gametes.
Random fertilisationany male gamete with any female gamete.

Inheritance

Inheritance genetic variation affects gamete DNA so it is inherited; environmental variation is not.

Part 10 of 22Natural selection and evolution

Selection pressures

Selection pressures biotic and abiotic (light, temperature, water, pH) factors.
- (predation, competition, disease)
Lions: competition for food and mates, drought and heat.

Process

Process individuals with phenotypes giving a selective advantage are more likely to survive and reproduce,
so favourable alleles rise in frequency and unfavourable ones fall.
Say "more likely to survive", not "guaranteed".

Evolution

Evolution formation of new species from pre-existing ones through changes in gene pools and allele frequencies.
Evolution is gradual change in a population; speciation is formation of a new species, usually after reproductive isolation,
- and is faster in organisms with short generation times.

Part 11 of 22Types of selection

Type	Effect	Example
Stabilising	Selects the average and removes extremes	Human birth weight
Directional	Shifts the population towards one extreme	Antibiotic resistance in bacteria
Disruptive	Selects both extremes and removes intermediates, maintaining two phenotypes (polymorphism)	Galapagos finch beak size: small or large beaks suit different seeds, medium beaks are selected against

Part 12 of 22Speciation and genetic drift

Reproductive isolation

Reproductive isolation stops gene flow.

Seasonalbreeding at different times.
Mechanicalstructures prevent mating.
Behaviouraldifferent courtship.

Genetic isolation

Genetic isolation each population accumulates its own allele changes until they cannot interbreed to produce fertile offspring.

Allopatric speciation

Allopatric speciation a geographical barrier splits the population; different selection pressures change each group.
- (mountain, river, road)

Sympatric speciation

Sympatric speciation no barrier; ecological separation or behavioural separation (feeding, courtship).
- (soil pH changing flowering time)
Lake fishbottom feeders with long jaws and open-water feeders with short jaws, with different courtship, become separate species.
Separate the cause of initial separation from the factor stopping breeding later.

Genetic drift

Genetic drift chance, not selection, decides which alleles are passed on;
- it matters most in small populations because chance events change allele frequencies quickly.
Examplepink-flower seeds landing on a rubber surface and failing to germinate.

Part 13 of 22Practical: bead model of allele frequency and drift

Put 24 red (R) and 24 white (r) beads in a container and mix.

Draw two beads at random to represent a mating and record the genotype.

Replace the beads and repeat 24 times.

Compare with the expected 1 RR : 2 Rr : 1 rr (25%, 50%, 25%).

Result

Result sample results differ because of chance.
- (5 RR, 7 Rr, 12 rr)
Alleles in the new population: red = 5 × 2 + 7 = 17; white = 12 × 2 + 7 = 31.
Allele frequencies: red 35.4%, white 64.6% (from 50:50), a change of 14.6%.

Second round

17 red and 31 white beads gave 4 RR, 5 Rr, 15 rr: red 13 (27.1%) and white 35 (72.9%), so the drift grows.
- (22.9% from the start)

Computer models

Computer models simulate selection pressures, mutation, dominance and environmental change, speeding up evolutionary time,
- and can show drift against selection.

Part 14 of 22Ecosystem terms and the niche

Ecosystem terms and the niche

Biotic factors

Biotic factors are living (predation, competition, disease, food); abiotic factors are non-living (temperature, light, pH, water, minerals).

Ecosystem

Ecosystem a community and its interactions with non-living factors, with energy flow and nutrient recycling;
- habitat — the local environment where a species lives.

Niche

Niche the role of an organism: resource use, responses to abiotic factors and interactions with biotic factors.
Each species has a unique niche; overlapping niches cause competition so one species wins or both decline.
Three warbler species feed at different heights in conifers (niche differentiation).

Part 15 of 22Carrying capacity and population limits

1Carrying capacity

Carrying capacity the maximum stable population size an ecosystem can support; a growth curve plateaus there.

2Abiotic limits

Abiotic limits light, water, temperature, space, soil pH.
Away from the optimum temperature, mammals spend more energy on homeostasis, leaving less for growth and reproduction.

3Interspecific competition

Interspecific competition grey squirrels outcompete red squirrels as they eat a wider range of food, are larger and store more winter fat.

4Intraspecific competition

Intraspecific competition rising numbers make resources limiting, so the population stabilises.

5Predator-prey cycle

Predator-prey cycle prey rise, predators rise, prey fall, predators fall, then repeats (lynx and hare).

Part 16 of 22Sampling and estimating population size

Sampling

Sampling random sampling avoids bias;
- systematic sampling can be biased by choice but suits environmental gradients.

Quadrat measures

Quadrat measures frequency = quadrats containing the species ÷ total quadrats × 100 ; percentage cover uses 100 sub-squares (89 squares gives 89%).
- (18 of 50 gives 36%)

Belt transect

Belt transect quadrats at regular intervals along a tape across a changing gradient, giving quantitative data.

Mark-release-recapture

Mark-release-recapture for motile animals: N = (n₁ × n₂) ÷ m₂.

n₁ marked and released, n₂ in the second sample, m₂ marked ones recaptured.
Leafhoppers236 × 244 ÷ 71 = 811.

Assumptions

Assumptions marked animals mix fully, marking does not affect survival, marks stay visible and the population size is constant.
- (no births, deaths or migration)

Part 17 of 22Succession and its management

Succession and its management

Succession

Succession gradual change in an ecosystem from simple to complex, with changing biotic and abiotic conditions.

Primary succession

Primary succession on bare rock

Pioneer species (moss, lichens) colonise; their decay forms basic soil (humus).
Small plants and grasses grow, deepening the soil and holding it.
Shrubs and small trees follow as soil and water improve.
Large trees form a climax communitystable and diverse.

Management for conservation

Management for conservation intermediate stages support species not found in climax communities.
- (grassland, heathland, moorland)
Mowing and grazing stop shrubs and trees establishing.
Managed burning removes woody plants and lets heather regrow.
Scottish moorland would naturally become spruce forest; management keeps both.

Part 18 of 22Growth of microorganisms and logarithmic scales

Turbidity

Turbidity as microorganisms multiply in broth, the culture becomes cloudier;
- a colorimeter, light sensor or turbidity meter measures light transmission at fixed times to plot a growth curve.

Log scales

Log scales increase by powers of 10 (10² = 100, 10³ = 1000), so the intervals on the axis are uneven but a huge range fits on one graph.
Reading valueslog 100 = 2, log 1000 = 3, so 320 lies near 2.5.
The pH scale is logarithmic.

Exam

Exam you interpret log graphs or explain their benefit, rather than draw them.

Part 19 of 22Conservation and human need

Method	How it manages conflict
National and marine parks	Legal restrictions on access, development and hunting balance biodiversity and land use
Public engagement and ecotourism	Income and jobs build community support
Zoos (captive breeding)	Restore populations and support research, relieving pressure on wild populations
Botanic gardens	Conserve rare plants and enable reintroduction
Frozen zoos and seed banks	Store genetic material and seeds for future restoration

Conflict
- land, water, wood and fuel needs damage ecosystems;
  - conservation means sustainable management and often managing succession.

Part 20 of 22Evaluating conservation data: crayfish

Study 1

Study 1 signal crayfish were removed after 2 years; native crayfish rose from 25 to 45 over 4 years,
while a control stream with no removal fell from 60 to 20.
Conclusioncompetition with the invader may cause the native decline.
The control, random sampling and long duration make it valid and unbiased.

Study 2

Study 2 a 20 m stream for 18 months; native crayfish fell from 25 to 13 and then 9 even after removal.
Conclusion conflicts with Study 1.
Weaknesses
- no control site, so water quality, a new predator or temperature may be responsible;
  - shorter duration.

Conflicting evidence

Conflicting evidence a single study is rarely enough for national conservation action;
- many similar studies are needed.

Part 21 of 22Practical: species distribution and abiotic factors

Choose the method
- random sampling with frame quadrats where no obvious gradient exists; systematic belt transects where a factor changes.
  - (altitude, shade to open field)

Lay two tape measures as axes for a 5 m by 5 m grid (random) or a 10 m transect (systematic).

Generate 10 random coordinates, or choose regular intervals such as every 1 m.

At each point place the quadrat and record percentage cover of the species.

Measure the abiotic factor (digital thermometer for soil temperature, pH meter, photometer for light).

Plot percentage cover (dependent) against the abiotic factor (independent).

Test correlation with Spearman's rank correlation.

Limitations
- correlation is not causation; other abiotic or biotic factors may explain the distribution, so further investigation is needed.
  - (soil pH, pollinators)

Part 22 of 22Worked examples

A heterozygous brown-eyed horse (Ee) is test crossed. Describe the expected offspring and what a result of all brown-eyed offspring would show about an unknown brown parent. [4]4 marks
In a population, 16% show a recessive phenotype. Calculate the frequency of heterozygotes. [3]3 marks
1. q² = 0.16, so q = 0.4.
2. p = 0.6.
3. 2pq = 2 × 0.6 × 0.4 = 0.48.
Observed offspring 90:20:25:5 (expected 9:3:3:1 from 140). Explain how to test whether the difference is significant. [4]4 marks
50 beetles were marked; a second sample of 60 contained 12 marked. Estimate the population. [2]2 marks
1. N = n₁ × n₂ ÷ m₂ = 50 × 60 ÷ 12.
2. N = 250.
Explain why genetic drift has a greater effect in small populations. [3]3 marks

Key terms

Term	Meaning
Codominance	Both alleles in a heterozygote are fully expressed.
Epistasis	One gene affecting the expression of another gene.
Gene pool	All alleles of all genes in an interbreeding population.
Hardy-Weinberg principle	Allele frequencies stay constant if specific conditions are met.
Disruptive selection	Selection for both extreme phenotypes and against intermediates.
Allopatric speciation	Speciation after a geographical barrier separates populations.
Genetic drift	Random change in allele frequencies, especially in small populations.
Carrying capacity	The maximum stable population size an environment can support.
Climax community	The stable final community in succession.

Avoid these mistakes

Writing "dominant means more common" — dominant alleles are expressed in heterozygotes but may be rare.

Using p and q confusingly — start from q², the homozygous recessive frequency.

Mixing up evolution and speciation — speciation is the formation of a new species.

Saying genetic drift is selection — it is random change, strongest in small populations.

Saying mark-recapture works for plants — it is used for motile animals; quadrats for sessile ones.

Last updated: 14 October 2026