What is Incomplete Dominance

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In this case, dominant allele do not completely express itself, resulting in a different phenotype. F1 generation expresses a phenotype which is intermediate between those of the parent.

Carl Correns discovered incomplete dominance and cytoplasmic dominance.


Incomplete dominance is a type of Gene interaction, in which, in a heterozygous condition, both alleles of a gene at a locus are partially expressed, often resulting in an intermediate (or different) phenotype. It is also called partial dominance.

Gene Interactions

It is a type of Gene Interaction. The other types are:
  • Intragenic (Allelic) Gene Interaction
    • Complete Dominance
    • Incomplete Dominance
    • Co-Dominance
    • Multiple Alleles
    • Lethal Genes
  • Intergenic (Non-Allelic) Gene Interaction
    • Epistasis
    • Duplicate Genes
    • Complementary Genes
    • Supplementary Genes
    • Polymeric Genes
    • Pleiotropy
    • Polygenic Genes

It is NOT an example of blending inheritance, although, in F1 generation, it seems to be so. It is not blending inheritance, because parental characters reappear in F2 generation.

The phenomenon was first studied in Mirabilis Jalapa (four o' clock, or marvel of Peru) plant. The phenotypic and genotypic monohybrid ratio in F2 generation is 1 : 2 : 1.

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It is an exception to Mendel's Law of Dominance

Mendel's Law of Dominance states that only one factor expresses itself in F1 generation.

F2 generation expresses both the dominant and the hidden recessive factor in the ratio of 3 : 1 in the monohybrid cross.

In a hybrid where both the contrasting alleles are present, only one factor/allele called dominant is able to express its effect, while the other factor/allele called recessive remains suppressed in F1 generation. This is law of dominance.

Incomplete Dominance and Co-Dominance are exceptions to Mendel's Law of Dominance.

Multiple Choice Questions

What are exceptions to Mendel`s law of dominance?
(1) Incomplete dominance
(2) Co-dominance
(3) both
(4) none

Ans: http://zoomgenetics.apphb.com/?id=m21

Who discovered Incomplete dominance and Cytoplasmic dominance
(1) Carl Correns
(2) Regnier de Graaf
(3) Carl Wilhelm von Nageli
(4) Archibald Garrod

Ans: http://zoomgenetics.apphb.com/?id=m28

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