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Codominance Knowledge-Based Study

Biology · Heredity · Biology · Non-Mendelian Genetics

Biology topic explanations on Codominance.

Study notes

  1. What is Codominance?

    In classical Mendelian genetics, a dominant allele completely masks the expression of a recessive allele in a heterozygous organism. However, many inheritance patterns do not follow this rule. Codominance is a form of non-Mendelian inheritance in which two different alleles for a specific gene are both fully and simultaneously expressed in the phenotype of a heterozygote.

    Unlike complete dominance, neither allele is dominant or recessive to the other. Unlike incomplete dominance, the heterozygous phenotype is not an intermediate blend of the parental traits. Instead, both parental phenotypes are distinctly observable side-by-side.

    Common Examples in Nature

    • Human MN Blood Group: Red blood cells can carry M antigens, N antigens, or both. An individual with genotype LMLM produces M antigens, while LNLN produces N antigens. A heterozygous individual with genotype LMLN produces both M and N antigens equally on their red blood cells.
    • ABO Blood Group (AB Phenotype): The gene for the ABO blood system includes multiple alleles (IA, IB, and i). The IA and IB alleles are codominant. A heterozygous individual with the genotype IAIB expresses both type A and type B carbohydrates on their erythrocytes, resulting in blood type AB.
    • Roan Coat Color in Cattle: When a purebred red bull (CRCR) mates with a purebred white cow (CWCW), the resulting offspring have the genotype CRCW. Their coat, known as roan, consists of individual red hairs and individual white hairs growing alongside each other rather than blended pink hairs.

    Analyzing Codominant Crosses

    When predicting the outcomes of crosses involving codominant traits, standard Punnett squares are used, but the resulting phenotypic ratios differ from classical Mendelian crosses.

    Consider a cross between two roan cattle (CRCW × CRCW):

    • Possible Genotypes: 1 CRCR : 2 CRCW : 1 CWCW
    • Genotypic Ratio: 1:2:1
    • Resulting Phenotypes: 1 Red : 2 Roan : 1 White
    • Phenotypic Ratio: 1:2:1

    A hallmark of both codominance and incomplete dominance is that the phenotypic ratio matches the genotypic ratio (1:2:1) in a monohybrid heterozygous cross because each genotype produces a distinct, identifiable phenotype.

    Distinguishing Codominance from Other Patterns

    To accurately identify the mode of inheritance, evaluate how alleles manifest in the heterozygous condition:

    • Complete Dominance: The heterozygote expresses only the dominant trait (e.g., Pp yields purple flowers). Phenotypic ratio is 3:1 in a monohybrid cross.
    • Incomplete Dominance: The heterozygote displays a blended, intermediate phenotype between both homozygotes (e.g., crossing red and white snapdragons produces pink offspring).
    • Codominance: The heterozygote displays both distinct parental traits simultaneously and fully without blending (e.g., blood type AB expresses both A and B antigens).