From monohybrid to epistasis

Genotype is the alleles present; phenotype is the expressed characteristic, and it is the product of genotype and environment.

• Monohybrid: Aa × Aa → 3 dominant : 1 recessive. A test cross against the homozygous recessive (aa) exposes an unknown genotype: any recessive offspring means the parent was heterozygous.
• Codominance: both alleles are expressed in the heterozygote. Blood group AB — I^AI^B — is the standard example, and it also illustrates multiple alleles (I^A, I^B, I^O), of which any one individual carries only two.
• Sex linkage: a gene on the X chromosome. Males are XY, so a single recessive allele on their one X is always expressed — they cannot be carriers. This is why haemophilia and red-green colour blindness are far commoner in males.
• Autosomal linkage: genes on the same chromosome are inherited together (unless separated by crossing over). Result: far more parental-type offspring and fewer recombinants than a 9:3:3:1 dihybrid ratio predicts.
• Epistasis: one gene masks or modifies the expression of another at a different locus. For example, if a gene producing a pigment precursor is homozygous recessive, no pigment is made at all — whatever the "colour" gene says. Ratios such as 9:3:4, 9:7 or 12:3:1 are the signature.

Always show the working: parental phenotypes, parental genotypes, gametes (circled), a Punnett square, offspring genotypes, then phenotypic ratio. Marks are for the method as much as the answer.

Sex linkage

A ratio that is not 9:3:3:1

The chi-squared test

Observed ratios almost never match expected ratios exactly. The chi-squared test answers one question: is the difference due to chance, or is something else going on?

• State the null hypothesis: there is no significant difference between observed and expected results.
• Calculate the expected numbers from the predicted ratio and the total.
• Work out χ², then find degrees of freedom = (number of classes − 1).
• Compare with the critical value at p = 0.05. If χ² is less than the critical value, the probability that the difference is due to chance is greater than 5%: accept the null hypothesis. If χ² is greater than or equal to the critical value, reject it — the difference is significant, and some other factor (linkage, epistasis, selection against a genotype) is at work.

Critical values at p = 0.05: 1 df → 3.84 · 2 df → 5.99 · 3 df → 7.82 · 4 df → 9.49. A dihybrid cross has four phenotype classes, so 3 degrees of freedom.

Your turn — chi-squared

Interpreting your chi-squared value

The Hardy-Weinberg principle

A gene pool is all the alleles of all the genes in a population. Allele frequency is the proportion of a particular allele in that pool. Hardy-Weinberg lets you calculate those frequencies from the phenotypes you can actually count.

The method is always the same: the recessive phenotype is the only genotype you can identify with certainty, so start there. Its frequency is q². Take the square root to get q, then p = 1 − q, then calculate 2pq for the carriers.

The principle only holds if: the population is large; mating is random; there is no selection for or against any genotype; there is no mutation; and there is no migration in or out. If a real population deviates from the predicted frequencies, one of these conditions has been broken — which is itself evidence that evolution is happening.

Your turn — Hardy-Weinberg

Selection, drift and speciation

Evolution is a change in allele frequency in a population over generations.

• Directional selection — one extreme is favoured, so the modal phenotype shifts. Antibiotic resistance is the classic case.
• Stabilising selection — the mean is favoured; both extremes are selected against; variation is reduced. Occurs when the environment is stable.
• Disruptive selection — both extremes are favoured over the mean, producing a bimodal distribution. It is the selection type most likely to lead to speciation.
• Genetic drift — allele frequencies change purely by chance, because only a sample of alleles is passed on. Its effect is large in small populations (including after a bottleneck or founder event) and negligible in very large ones.

Speciation requires reproductive isolation so that two gene pools stop mixing; mutation, selection and drift then act independently on each, and the populations diverge until they can no longer interbreed to produce fertile offspring.

• Allopatric — a geographical barrier (river, mountain range, ocean) separates the populations, which then experience different selection pressures.
• Sympatric — isolation arises within the same area, through behavioural, seasonal or mechanical barriers (or, in plants, polyploidy).

Carrying capacity, competition and predation

A population is all the organisms of one species in a habitat; a community is all the populations. The ecosystem includes the abiotic environment too. Niche: the role of a species — and no two species can occupy exactly the same niche in the same habitat indefinitely.

• A population introduced to a new habitat shows a lag phase (few individuals, acclimatising), then a log/exponential phase (resources plentiful), then a stationary phase as it reaches the carrying capacity — the maximum the habitat can sustain.
• Carrying capacity is set by abiotic factors (temperature, light, water, pH, mineral availability) and biotic factors: interspecific competition (between species — the poorer competitor may be excluded), intraspecific competition (within a species — this is what actually holds the population at the carrying capacity), and predation.
• Predator-prey cycles: prey numbers rise → more food for predators → predator numbers rise → prey are eaten and numbers fall → predators starve and fall → prey recover. The predator peak always lags behind the prey peak. In reality, disease and food supply usually matter more than predation alone.

Succession and conservation

Primary succession begins on bare rock or newly exposed ground with no soil.

• Pioneer species (e.g. lichens, algae) colonise. They are adapted to harsh abiotic conditions and can survive with almost no soil.
• They weather the rock, and when they die and are decomposed, they add humus, forming a thin soil that can retain water.
• This makes the environment less hostile, so new species (mosses, then grasses, then shrubs) can colonise. Crucially, each stage makes the environment less suitable for the previous species, which is out-competed and disappears.
• Biodiversity, biomass and soil depth all increase at each stage, until a stable climax community is reached (in the UK, usually deciduous woodland).

Deflected succession (a plagioclimax): human activity — grazing sheep, mowing, burning heather — halts succession before the climax. That is why a grassland or a heather moor persists. Conservation is therefore often active management: it means deliberately preventing succession, not simply leaving an area alone.

Sampling: use random quadrats (via random number coordinates) to avoid bias when estimating abundance in a uniform area; use a belt or line transect when there is an environmental gradient. Record percentage cover for plants, or frequency. For motile animals, use mark-release-recapture.

Your turn — mark-release-recapture

Why the pioneers disappear

Conservation and the human footprint

Conservation is the active management of ecosystems to maintain biodiversity, and it must be distinguished from preservation, which means leaving an area untouched. Because most British habitats are plagioclimaxes, doing nothing would simply allow succession to proceed and would destroy the very community you are trying to protect.

• Why bother? Ethical — organisms have a right to exist. Economic — species are sources of medicines, foods and genes for crop improvement; ecosystems provide services (pollination, flood control, soil stability). Aesthetic and cultural — landscape, recreation, tourism.
• Techniques: seed banks and captive breeding; protected areas and legislation; controlled grazing, coppicing and rotation; restocking and reintroduction; and education.
• Managing a conflict: conservation and farming compete for land. Compromises include maintaining hedgerows and field margins, leaving set-aside land, restricting the use of pesticides, and using crop rotation to reduce fertiliser demand.

Sustainable exploitation is the goal: harvesting at a rate that allows the population to replace itself. For a forest, that means coppicing and selective felling rather than clear-cutting; for a fishery, quotas, minimum mesh sizes and closed seasons during breeding.

Which type of selection?

Tap a statement, then tap the type of selection it describes.

Hardy-Weinberg terms

Tap an item on the left, then its partner on the right.

The big ideas to take away

That is the whole of AQA 3.7 Genetics, populations, evolution and ecosystems. Press Finish to see your score.