Epistasis

Spec Mapping — OCR H420 Module 6.1.2 — Patterns of inheritance, content statements covering epistasis as a deviation from independent-assortment Mendelian ratios in dihybrid crosses, including recessive, dominant and complementary epistasis (refer to the official OCR H420 specification document for exact wording). Epistasis joins linkage as one of the two key reasons that observed dihybrid ratios deviate from 9:3:3:1; the chi-squared test you have already met provides the formal tool to assess whether observed deviations are statistically significant.

The historical insight comes from William Bateson and Reginald Punnett (1905, paraphrased), who in their early dihybrid experiments with comb shape in chickens noticed F2 ratios that summed to 16 but were not 9:3:3:1. They proposed that the four classes had been merged — two phenotypes that looked identical despite different genotypes. The principle, christened epistasis (from Greek "standing upon"), is now a cornerstone of genetic analysis: when genes function in linked biochemical pathways or developmental cascades, one gene's product can mask or modify the expression of another.

Mendelian ratios assume that each gene acts independently of the others. In reality, many phenotypes are the result of several genes acting together, and sometimes the alleles at one locus mask or modify the expression of alleles at another. This phenomenon is called epistasis, and it is a major reason why dihybrid ratios in nature often deviate from the classic 9:3:3:1. OCR A-Level Biology A specification module 6.1.2(e) requires you to understand epistasis and to work out modified phenotypic ratios in dihybrid crosses.

Key Definitions:

• Epistasis — the interaction of different genes at different loci where one gene (the epistatic gene) masks or modifies the effect of another (the hypostatic gene).
• Recessive epistasis — masking occurs only when the epistatic gene is homozygous recessive. Modified ratio: 9 : 3 : 4.
• Dominant epistasis — masking occurs in the presence of even one dominant allele at the epistatic locus. Modified ratio: 12 : 3 : 1.
• Complementary epistasis — two genes must both be expressed for a phenotype to appear. Modified ratio: 9 : 7.

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Why Mendelian Ratios Break Down

A dihybrid cross between two heterozygotes (AaBb × AaBb) should give a 9:3:3:1 phenotypic ratio, as the Punnett square below shows:

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

Sixteen equally likely outcomes group into:

• 9 A_B_ (at least one dominant at each locus)
• 3 A_bb (dominant at A, recessive at B)
• 3 aaB_ (recessive at A, dominant at B)
• 1 aabb (recessive at both)

Epistasis combines some of these classes together, because the visible phenotypes overlap.

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Recessive Epistasis (9:3:4)

When the homozygous recessive genotype at one locus masks the phenotype at the other.

Coat colour in mice (the agouti example)

Mouse coat colour depends on two genes:

• Gene C — if cc, no pigment is produced at all → albino, regardless of the other gene.
• Gene A — in a pigmented mouse (C_), A_ gives agouti (brown with dark tips); aa gives black.

The cc genotype is epistatic to the agouti/black gene — it blocks the earlier step in the pigment pathway. A cc mouse cannot make any pigment at all, so whatever alleles it has at the A locus are irrelevant.

Cross two dihybrid mice CcAa × CcAa:

• 9 C_A_ — agouti
• 3 C_aa — black
• 3 ccA_ — albino (cannot make pigment)
• 1 ccaa — albino (cannot make pigment)

The two albino classes merge: 9 agouti : 3 black : 4 albino — the classic 9:3:4 recessive epistasis ratio.

flowchart LR
    CCAA[CcAa x CcAa] --> P1[9 agouti]
    CCAA --> P2[3 black]
    CCAA --> P3[3 albino cc A_]
    CCAA --> P4[1 albino cc aa]
    P3 --> M[Merged 4 albino]
    P4 --> M

Why the merging makes biological sense

The two genes act sequentially in a biochemical pathway. Gene C catalyses the first step — without a functional C enzyme, no pigment precursor is made, and gene A has nothing to modify. Blocking an early step makes later steps irrelevant: that is the essence of recessive epistasis.

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Dominant Epistasis (12:3:1)

When a single dominant allele at one locus masks the phenotype at the other.

Squash fruit colour

In summer squash, colour depends on two genes:

• Gene W — W_ (dominant) gives white fruit, regardless of the other gene.
• Gene Y — in ww squash, Y_ gives yellow and yy gives green.

Gene W is dominant-epistatic: its dominant allele produces an inhibitor of pigment production, so W_ squash are white no matter what alleles they have at the Y locus.

Cross two dihybrids WwYy × WwYy:

• 9 W_Y_ — white
• 3 W_yy — white
• 3 wwY_ — yellow
• 1 wwyy — green

The two "W_" classes merge: 12 white : 3 yellow : 1 green — the classic 12:3:1 dominant epistasis ratio.

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Complementary Epistasis (9:7)

When dominant alleles at BOTH loci are required for any expression at all.

Flower colour in sweet peas

Cross two white-flowered sweet pea strains (both from different breeds) and the F1 are all purple. When the F1 are crossed, the F2 show 9 purple : 7 white. Why?

• Purple pigment is produced by a two-step pathway: a colourless precursor is converted to an intermediate by enzyme A, and the intermediate is converted to purple pigment by enzyme B.
• Both enzymes must be functional for purple to appear.
• Gene A and gene B must each have at least one dominant allele.
• 9 A_B_ → purple
• 3 A_bb + 3 aaB_ + 1 aabb → white (missing either or both enzymes)
• Ratio: 9 purple : 7 white.

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Summary Table of Modified Ratios

Type of epistasis	Mechanism	Modified ratio
None (independent)	Alleles act independently	9 : 3 : 3 : 1
Recessive epistasis	Homozygous recessive at one locus masks the other	9 : 3 : 4
Dominant epistasis	Dominant allele at one locus masks the other	12 : 3 : 1
Complementary (duplicate recessive)	Both loci must have a dominant allele	9 : 7

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How to Identify Epistasis in an Exam Question

• You are told about two genes that interact.
• The observed phenotypic ratio does not fit 9:3:3:1.
• The ratio sums to 16 — all the classes are still there, but they have been merged.
• Work out which classes have merged and why. Usually there is a biochemical pathway in the background: one gene is "upstream" of the other, or both are needed together.

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Worked Example: Coat Colour in Labrador Retrievers

Labradors have three coat colours: black, chocolate and yellow. Two genes are involved:

• Gene B — B_ black pigment, bb chocolate.
• Gene E — E_ pigment deposited in hair, ee pigment not deposited → yellow regardless of B genotype.

The ee genotype is epistatic to the B gene. Cross two dihybrid black Labradors (BbEe × BbEe):

• 9 B_E_ — black
• 3 bbE_ — chocolate
• 3 B_ee — yellow
• 1 bbee — yellow

Ratio: 9 black : 3 chocolate : 4 yellow — recessive epistasis, because the ee (homozygous recessive at E) is doing the masking.

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Exam Tip

If an exam question gives you a modified ratio (e.g. 9:3:4 or 12:3:1), do not just recite "this is epistasis". Explain why by describing the biochemical pathway: which gene is upstream, which is masked, and what makes the classes overlap. The marks are often for the mechanistic explanation, not the label.

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Common Exam Mistakes

• Confusing epistasis with linkage. Epistasis involves two genes on (usually) different loci whose gene products interact. Linkage is about physical proximity on the same chromosome.
• Thinking epistasis only gives 9:3:4. There are several modified ratios — know at least 9:3:4, 12:3:1 and 9:7.
• Forgetting that the ratio still sums to 16. If you write "9:3:3" you have missed a class.
• Saying "the epistatic gene is always dominant". It can be either. Recessive epistasis requires the homozygous recessive to mask.
• Not linking epistasis to pathways. The biological reason is usually that one gene acts before the other in a biochemical pathway, so blocking the earlier step makes the later step irrelevant.
• Using epistasis as a synonym for "dominance". Dominance is between alleles at the same locus; epistasis is between genes at different loci.

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Quick Recap

• Epistasis is when one gene masks or modifies the effect of another gene at a different locus.
• Recessive epistasis (e.g. albino mice, yellow Labradors) gives a 9:3:4 ratio.
• Dominant epistasis (e.g. white summer squash) gives a 12:3:1 ratio.
• Complementary epistasis (e.g. sweet pea flower colour) gives a 9:7 ratio.
• Modified ratios still sum to 16 out of a dihybrid cross — classes are merged, not lost.
• Epistasis usually reflects a biochemical pathway in which one gene acts upstream of another.

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Recessive Epistasis Biochemical Logic — Inline SVG

The biochemical logic of recessive epistasis is best seen as a sequential pathway in which the upstream gene controls a substrate that the downstream gene further modifies.

Pathway: Tyrosine

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<text x="130" y="12" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="10" fill="#222">C</text>
<text x="130" y="35" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="10" fill="#444">tyrosinase</text>

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<text x="290" y="12" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="10" fill="#222">A</text>
<text x="290" y="35" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="10" fill="#444">agouti gene</text>

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Genotype-phenotype outcomes: CcAa × CcAa → 9 C_A_ = agouti (pathway intact, agouti gene functional) 3 C_aa = black (pathway intact, no agouti banding)

3 ccA_ = albino (cc blocks tyrosinase — no pigment) 1 ccaa = albino (same reason — A is irrelevant) Combined: 4 albino → ratio 9:3:4

The crucial principle: cc blocks step 1, so anything that gene A would have done downstream becomes irrelevant. The two "albino" genotype classes (3 ccA_ and 1 ccaa) merge into a single phenotypic class, giving the 9:3:4 ratio. Whenever you see a ratio summing to 16 with one class merged, ask: which gene is upstream in the biochemical pathway?

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Comparison Table of Modified Ratios