Mendelian Genetics & Punnett Squares
Explore different patterns of inheritance through Punnett square analysis
1. Simple Dominance (Complete Dominance)
Cross: Aa × Aa
Example: Pea plant height (T = tall, t = short)
Genotypic Ratio:
1 AA : 2 Aa : 1 aa
Phenotypic Ratio:
3 Dominant (Tall) : 1 Recessive (Short)
Key Point: Dominant allele (A) completely masks the recessive allele (a) in heterozygotes. AA and Aa both show the dominant phenotype.
2. Codominance
Cross: CRCW × CRCW
Example: Cattle coat color (R = red, W = white)
Genotypic Ratio:
1 CRCR : 2 CRCW : 1 CWCW
Phenotypic Ratio:
1 Red : 2 Roan : 1 White
Key Point: Both alleles are expressed equally in heterozygotes. CRCW produces roan coat (red and white hairs mixed), showing both colors simultaneously.
3. Incomplete Dominance
Cross: RRRW × RRRW
Example: Snapdragon flower color (R = red, W = white)
Genotypic Ratio:
1 RRRR : 2 RRRW : 1 RWRW
Phenotypic Ratio:
1 Red : 2 Pink : 1 White
Key Point: Heterozygotes show an intermediate phenotype (blend). RRRW produces pink flowers, a blend between red and white. Neither allele is fully dominant.
4. Dihybrid Cross (Two Traits, Simple Dominance)
Cross: AaBb × AaBb
Example: Pea plants (A=round, a=wrinkled; B=yellow, b=green)
Genotypic Ratio:
9 different genotypes in complex ratio
Phenotypic Ratio: 9:3:3:1
- 9 A_B_ (Round Yellow)
- 3 A_bb (Round Green)
- 3 aaB_ (Wrinkled Yellow)
- 1 aabb (Wrinkled Green)
Key Point: Two genes assort independently. The 9:3:3:1 ratio is characteristic of dihybrid crosses with simple dominance at both loci.
5. Epistasis (Gene Interaction)
Cross: BbEe × BbEe
Example: Labrador retriever coat color
B gene: B_=Black, bb=Brown
E gene: E_=deposits pigment, ee=no pigment (yellow)
Genotypic Ratio:
Standard 9:3:3:1 for two genes
Phenotypic Ratio: 9:3:4
(Modified by epistasis)
- 9 B_E_ = Black
- 3 bbE_ = Brown
- 4 _ _ee = Yellow
Key Point: The E gene is epistatic to the B gene. When ee is present, no pigment is deposited regardless of B genotype, resulting in yellow coat. This modifies the expected 9:3:3:1 ratio to 9:3:4.
Summary of Inheritance Patterns
Simple Dominance
One allele completely masks the other. Aa shows dominant phenotype.
Codominance
Both alleles expressed equally. Heterozygote shows both traits simultaneously.
Incomplete Dominance
Heterozygote shows intermediate (blended) phenotype between two homozygotes.
Dihybrid Cross
Two genes on different chromosomes assort independently. Classic 9:3:3:1 ratio.
Epistasis
One gene masks or modifies the expression of another gene. Modifies expected ratios.