Incomplete Dominance: How Blended Inheritance Works in Genetics

Incomplete dominance is an inheritance pattern where neither allele is completely dominant, resulting in a heterozygous phenotype that is intermediate between the two homozygous phenotypes.

Classic Example: Snapdragon Flower Color

• RR (homozygous red) × rr (homozygous white) → All Rr (heterozygous pink) offspring
• Rr × Rr cross: 1 RR (red) : 2 Rr (pink) : 1 rr (white) — phenotypic ratio matches genotypic ratio (1:2:1)

Key Characteristics

• Heterozygote shows a distinct, blended phenotype—not a mix of both traits, but a true intermediate.
• Both alleles are expressed at reduced levels or produce partially functional products.
• No allele masks the other; dosage effect determines phenotype.

Other Examples

• Human hair texture: Curly (CC) × Straight (cc) → Wavy (Cc)
• Familial hypercholesterolemia: Normal (NN) × Homozygous affected (HH) → Heterozygous (NH) with intermediate cholesterol levels
• Tay-Sachs disease carriers: Intermediate hexosaminidase A enzyme activity
• Andalusian chickens: Black (BB) × White (bb) → Blue (Bb)

Contrast with Codominance

In codominance (e.g., ABO blood groups), both alleles are fully and distinctly expressed in the heterozygote (type AB blood has both A and B antigens). In incomplete dominance, the heterozygote shows a novel intermediate phenotype (pink, not red-and-white spotted).

Understanding incomplete dominance is essential for genetic counseling, breeding programs, and interpreting non-Mendelian inheritance patterns in medicine and agriculture.