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Exceptions to Mendel's Principles > Incomplete Dominance

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Estimated time: 3 minutes
CBSE: Class 12
CISCE: Class 12

Definition: 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.

Maharashtra State Board: Class 12

Key Points: Exceptions to Mendel's Principles > Incomplete Dominance

  • 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.

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