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Mendel's analysis of pea-plant genetics gave us discrete categories: round vs wrinkled, yellow vs green, tall vs dwarf. Most real traits — height, skin colour, intelligence, agricultural yield — show continuous variation that defies neat phenotypic ratios. The bridge between Mendelian (single-gene, discrete) and continuous (multi-gene, blended) inheritance is built from two phenomena: epistasis, in which one gene modifies the expression of another at a different locus, producing modified dihybrid ratios that depart from 9:3:3:1; and polygenic inheritance, in which multiple genes each contribute small additive effects to a single quantitative characteristic, producing a normal distribution of phenotypes shaped by environment as well as genotype. Together these mechanisms explain why most heritable traits do not show Mendelian ratios and why the genetic architecture of common diseases (diabetes, coronary heart disease, schizophrenia) involves many genes of small individual effect.
Spec mapping: This lesson sits in AQA 7402 Section 3.7.1 — Inheritance, with strong synoptic links to Section 3.8 (control of gene expression) and Section 3.7.2 (selection on continuous variation). The relevant content covers epistasis (the interaction of two genes at different loci), polygenic inheritance and continuous variation, the relationship between genotype, environment and phenotype, and the modified dihybrid ratios that result from epistasis. (Refer to the official AQA specification document for exact wording.)
Key Definition: Epistasis occurs when the allele(s) of one gene (the epistatic gene) affect or mask the phenotypic expression of allele(s) at another gene (the hypostatic gene) at a different locus. Epistasis produces modified dihybrid ratios — deviations from the standard 9:3:3:1 ratio.
Important distinctions:
In recessive epistasis, the homozygous recessive genotype at one locus masks the expression of the other gene.
Two genes control flower colour in a plant species:
Biosynthetic pathway: Precursor → (Gene 1: C allele needed) → Colourless intermediate → (Gene 2: P allele needed) → Purple pigment
Cross: CcPp × CcPp
| Genotype class | Proportion | Phenotype | Explanation |
|---|---|---|---|
| C_P_ | 9/16 | Purple | Both enzymes functional; pigment produced |
| C_pp | 3/16 | White | Intermediate produced but not converted to pigment |
| ccP_ | 3/16 | White | No intermediate produced; gene 2 has no substrate |
| ccpp | 1/16 | White | Neither enzyme functional |
Modified ratio: 9 purple : 7 white
The cc genotype is epistatic — it masks the effect of gene 2 because no intermediate is produced for gene 2 to act upon. This is also called complementary gene interaction because both dominant alleles must be present for the purple phenotype.
Two genes control coat colour in Labradors:
Cross: BbEe × BbEe
| Genotype class | Proportion | Phenotype |
|---|---|---|
| B_E_ | 9/16 | Black |
| bbE_ | 3/16 | Chocolate (brown) |
| B_ee | 3/16 | Yellow |
| bbee | 1/16 | Yellow |
Modified ratio: 9 black : 3 chocolate : 4 yellow
Here, the ee genotype is epistatic to gene B — when ee is present, no pigment is deposited regardless of genotype at the B locus. The 3/16 (B_ee) and 1/16 (bbee) classes both produce yellow, combining to give 4/16 yellow.
In dominant epistasis, a dominant allele at one locus masks the expression of the other gene.
Two genes control fruit colour in squash:
Cross: WwYy × WwYy
| Genotype class | Proportion | Phenotype |
|---|---|---|
| W_Y_ | 9/16 | White |
| W_yy | 3/16 | White |
| wwY_ | 3/16 | Yellow |
| wwyy | 1/16 | Green |
Modified ratio: 12 white : 3 yellow : 1 green
The W allele is epistatic — its presence masks the expression of gene Y. Only when the genotype is ww can gene Y be expressed.
| Type of Epistasis | Modified Ratio | Explanation |
|---|---|---|
| Complementary (recessive epistasis) | 9:7 | Both dominant alleles needed for one phenotype |
| Recessive epistasis | 9:3:4 | Homozygous recessive at one locus masks other gene; hypostatic gene has distinguishable phenotypes |
| Dominant epistasis | 12:3:1 | Dominant allele at one locus masks other gene |
| Duplicate recessive epistasis | 9:6:1 | Homozygous recessive at either locus produces the same phenotype |
| Duplicate dominant epistasis | 15:1 | Dominant allele at either locus produces the same phenotype |
| Inhibitory epistasis | 13:3 | Dominant allele at one locus inhibits expression |
Exam Tip: If a dihybrid cross produces a ratio that does not fit 9:3:3:1, add the numbers and check whether they sum to 16. If they do, epistasis is likely. Identify which phenotypic classes have been combined to produce the modified ratio.
The summary table lists duplicate dominant epistasis (15:1) and duplicate recessive epistasis (9:6:1), but these are worth working through because they reveal how two genes with overlapping function generate their ratios — a favourite discriminator at the top of the mark range.
Grain colour in some cereals is produced by two independent genes, A and B, either of which alone is sufficient to make pigment. A dominant allele at either locus gives coloured grain; only the double recessive is colourless.
Cross: AaBb × AaBb
| Genotype class | Proportion | Phenotype | Reason |
|---|---|---|---|
| A_B_ | 9/16 | Coloured | Both genes active |
| A_bb | 3/16 | Coloured | Gene A alone suffices |
| aaB_ | 3/16 | Coloured | Gene B alone suffices |
| aabb | 1/16 | Colourless | Neither functional allele present |
The three coloured classes (9 + 3 + 3) combine to give 15 coloured : 1 colourless. Because the two genes are functionally redundant, a plant needs to fail at both loci to lose the phenotype — which is why the colourless class is so rare. This redundancy is biologically important: duplicated genes buffer an organism against loss-of-function mutations, and the 15:1 ratio is the genetic fingerprint of that buffering.
Now suppose the two genes contribute additively to a quantitative feature such as fruit shape, where each active locus adds one increment of effect. Having a dominant allele at both loci gives one extreme phenotype, at one locus gives an intermediate, and at neither gives the other extreme.
Cross: AaBb × AaBb
| Genotype class | Proportion | Phenotype |
|---|---|---|
| A_B_ | 9/16 | Disc-shaped (both loci active) |
| A_bb + aaB_ | 6/16 | Round (one locus active) |
| aabb | 1/16 | Elongate (neither locus active) |
Here the two single-active-locus classes are phenotypically identical, combining to 9 : 6 : 1. The distinction from the 9:3:4 pattern is subtle and often examined: in 9:6:1 the two genes make equivalent, interchangeable contributions, whereas in 9:3:4 one gene is strictly epistatic to (masks) the other. Reading the ratio backwards to the biology is the skill being tested.
Because polygenic traits are shaped by both genes and environment, the proportion of phenotypic variation attributable to genetic differences — the broad-sense heritability — is a key quantitative-genetics measure. Suppose a population of a plant shows a total phenotypic variance in height of V_P = 40 (arbitrary units). A genetically uniform (clonal) line grown in the same field shows a variance of V_E = 10, which must be entirely environmental because the genotype is constant. The genetic variance is therefore V_G = V_P − V_E = 40 − 10 = 30, and broad-sense heritability H² = V_G / V_P = 30/40 = 0.75.
The interpretation is precise and easily misstated: 75% of the variation in height in this population, in this environment is attributable to genetic differences. It does not mean any individual plant is "75% genetic", and the same trait measured in a more variable environment would give a lower heritability because V_E — and hence V_P — would be larger. This population-and-environment dependence is exactly the A*-level nuance that distinguishes a sophisticated answer from a mechanical one.
By the end of this lesson you should be able to: define epistasis and distinguish it from dominance; derive and interpret the modified dihybrid ratios (9:7, 9:3:4, 12:3:1, 15:1, 9:6:1, 13:3) from the underlying biology; explain polygenic inheritance and continuous variation; partition phenotypic variance into genetic and environmental components and calculate broad-sense heritability; and distinguish pleiotropy from polygenic inheritance.
Key Definition: Polygenic inheritance occurs when a single characteristic is controlled by two or more genes, each contributing a small additive effect to the phenotype. Polygenic traits typically show continuous variation.
Skin colour is controlled by at least 3–4 genes (simplified model uses 3 genes: A, B, C):
With 3 genes, there are 7 phenotypic classes (0–6 contributing alleles), and the frequency distribution follows a binomial pattern that approximates a normal distribution.
The phenotype of a polygenic trait is influenced by both genotype and environment:
Key Point: Continuous variation in polygenic traits results from the combined effects of multiple genes and environmental influences. This contrasts with discontinuous variation, which produces distinct categories (e.g., blood group, tongue rolling) and is typically controlled by one or two genes with little environmental influence.
| Feature | Single-gene trait | Polygenic trait |
|---|---|---|
| Number of genes | One (or two for dihybrid) | Multiple (three or more) |
| Type of variation | Discontinuous (distinct categories) | Continuous (range of values) |
| Distribution | Discrete ratios (e.g., 3:1, 9:3:3:1) | Normal distribution (bell curve) |
| Environmental effect | Usually minimal | Often significant |
| Examples | ABO blood group, cystic fibrosis | Height, skin colour, intelligence |
The term "epistasis" was coined by William Bateson in 1907 to describe the interactions he observed between genes affecting comb shape in chickens and flower colour in peas. Bateson was one of the rediscoverers of Mendel's work (along with de Vries and Correns) and an enthusiastic populariser; he also coined the word "genetics" itself. His studies of comb shape — pea, rose, walnut, single, in the ratio 9:3:3:1 from a dihybrid cross between two double heterozygotes — were among the first demonstrations that two genes can interact to produce novel phenotypes that neither alone could generate.
Bateson's "complementary" gene interaction (now classified as recessive epistasis, 9:7) is the canonical case study: two pure-breeding white flower lines crossed together produce all-purple F₁ offspring, with F₂ showing a 9:7 ratio of purple to white. The interpretation requires two genes each contributing one essential step of a biosynthetic pathway — neither alone can produce the pigment, but both together can.
This lesson connects to several other AQA 7402 specification sections:
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