Biology - Punnett Square Rules

Only works with Mendels Rules

Monohybrid Cross (1 Gene)

• Cross: A/a × A/a

• Gametes per parent: 2 → A or a

• Offspring: 2 × 2 = 4 boxes

• Phenotypes: 2 → Dominant (A_) or recessive (aa)

• Genotypes: 3 → AA, Aa, aa

Dihybrid Cross (2 Genes)

• Cross: A/a ; B/b × A/a ; B/b

• Gametes per parent: 2² = 4 → AB, Ab, aB, ab

• Offspring: 4 × 4 = 16 boxes

• Phenotypes: 2 × 2 = 4

• Genotypes: 3 × 3 = 9

Trihybrid Cross (3 Genes)

• Cross: A/a ; B/b ; C/c × A/a ; B/b ; C/c

• Gametes per parent: 2³ = 8

• Offspring: 8 × 8 = 64 boxes

• Phenotypes: 2 × 2 × 2 = 8

• Genotypes: 3 × 3 × 3 = 27

Tetrahybrid Cross (4 Genes)

• Cross: A/a ; B/b ; C/c ; D/d × A/a ; B/b ; C/c ; D/d

• Gametes per parent: 2⁴ = 16

• Offspring: 16 × 16 = 256 boxes

• Phenotypes: 2⁴ = 16

• Genotypes: 3⁴ = 81

Key Rules

• Gametes per parent: 2ⁿ

• Offspring (Punnett squares): 4ⁿ

• Phenotypes: 2ⁿ

• Genotypes: 3ⁿ

Memory Tricks

“n = number of genes” trick: Think of rolling dice

• Gametes = 2 sides per die

• Offspring = 2 dice per parent → 4 sides per gene → 4ⁿ

• Phenotypes = 2 outcomes per die → 2ⁿ

• Genotypes = 3 outcomes per die → 3ⁿ