Dihybrid Inheritance and Epistasis

This lesson covers the inheritance of two genes simultaneously (dihybrid inheritance) and interactions between genes (epistasis), as required by the Edexcel A-Level Biology specification (9BI0, Topic 8).

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What Is Dihybrid Inheritance?

Dihybrid inheritance involves the simultaneous inheritance of two genes located on different chromosomes (or far apart on the same chromosome, so they assort independently).

Mendel studied dihybrid crosses in pea plants and established the Law of Independent Assortment: the alleles of one gene segregate independently of the alleles of another gene during gamete formation (provided the genes are on different chromosomes).

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Standard Dihybrid Cross

Cross: AaBb × AaBb

Each parent produces four types of gametes in equal proportions: AB, Ab, aB, ab

Punnett Square (4 × 4)

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

Expected Phenotype Ratio

With complete dominance at both loci: