You are viewing a free preview of this lesson.
Subscribe to unlock all 12 lessons in this course and every other course on LearningBro.
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.
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:
Epistasis combines some of these classes together, because the visible phenotypes overlap.
When the homozygous recessive genotype at one locus masks the phenotype at the other.
Mouse coat colour depends on two genes:
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:
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
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.
When a single dominant allele at one locus masks the phenotype at the other.
In summer squash, colour depends on two genes:
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:
The two "W_" classes merge: 12 white : 3 yellow : 1 green — the classic 12:3:1 dominant epistasis ratio.
Subscribe to continue reading
Get full access to this lesson and all 12 lessons in this course.