Exceptions to Mendel's Principles > Incomplete Dominance

Incomplete dominance is the inheritance pattern in which neither allele of a gene is completely dominant over the other, so the heterozygous individual shows an intermediate phenotype between the two parental traits.

"A form of gene interaction where neither allele completely dominates the other, and the heterozygous individual shows an intermediate (in-between) phenotype."

• Also called Partial Dominance or Semi-dominance
• It is a non-Mendelian pattern - it violates Mendel's Law of Dominance
• Mendel's own pea experiments showed complete dominance; incomplete dominance was discovered later in other plants

Why Does It Happen?

• Each allele produces a certain amount of protein/enzyme
• In a homozygous dominant (RR): full protein → full colour
• In a heterozygous (Rr): only half the protein → intermediate colour
• In a homozygous recessive (rr): no functional protein → no colour​

Step 1: Parental Cross (P)

Cross: RR × rr​

Step 2: F₁ Generation

All offspring → Rr → Pink flower (intermediate)

F₁ result: 100% Pink (Rr)

Pink is neither red nor white - it is intermediate. This proves incomplete dominance.​

Step 3: F₂ Generation (F₁ × F₁ selfing)

Cross: Rr × Rr​

Genotypic Ratio = 1 : 2 : 1
Phenotypic Ratio = 1 : 2 : 1

Results of monohybrid cross in the plant Snapdragon, where one allele is incompletely dominant over the other allele

• Incomplete Dominance - Exception to law of dominance; neither allele is completely dominant; F₁ hybrid shows an intermediate expression of both characters.
• Example - Red (RR) × White (rr) in Mirabilis jalapa → F₁ offspring are Pink (Rr); neither red nor white dominates completely.
• F₂ Generation - Selfing of F₁ (Rr × Rr) gives:
  Genotypic ratio - 1RR : 2Rr : 1rr
  Phenotypic ratio - 1 Red : 2 Pink : 1 White
• Both phenotypic and genotypic ratios are 1:2:1 (unlike Mendel's 3:1 phenotypic ratio), which is the key difference from complete dominance.