Inheritance

In the snapdragon, Antirrhinum majus, a cross between a homozygous white-flowered plant (C^WC^W) and a homozygous red-flowered plant (C^RC^R) will produce offspring with pink flowers (C^WC^R).

This type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes is incomplete dominance.

•Codominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual

•Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect

•When representing alleles, the convention is to use superscripts for the different co-dominant alleles (recessive still lower case)

•An example of co-dominance is feathering in chickens – black (C^B) and white (C^W) feathers create a speckled coat (C^BC^W).

Codominance in Blood Groups

•Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen).

•ABO blood groups are controlled by a single gene with multiple alleles (A, B, O).

•The A, B and O alleles all produce a basic antigen on the surface of red blood cells.

•The A and B alleles are codominant and each modify the structure of the antigen to produce different variants.

•The O allele is recessive and does not modify the basic antigenic structure.

•When representing blood group alleles, the letter I is used to represent the different antigenic forms (isoagglutinogen).

•A allele = I^A  ;  B allele = I^B  ;  O allele = i (recessive).