Extensions of Mendelian Genetics (Deviation from Mendelism) - Intragenic Interactions - Codominance

Codominance:

• This pattern occurs due to simultaneous (joint) expression of both alleles in the heterozygote - The phenomenon in which two alleles are both expressed in the heterozygous individual is known as codominance.
• Example - Red and white flowers of Camellia, inheritance of sickle cell haemoglobin, ABO blood group system in human beings.

Examples:

1) A good example is different types of red blood cells that determine ABO blood grouping in human beings. In human beings, I^A and I^B alleles of I gene are codominant which follows Mendel's law of segregation.

ABO blood groups are controlled by gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles I^A, I^B, and i. The alleles I^A and I^B produce a slightly different form of sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. I^A and I^B are completely dominant over i, in other words when I^A and i are present only I^A expresses (because i does not produce any sugar), and when I^B and i are present I^B expresses. But when I^A and I^B are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars. Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes of the human ABO blood types.

2) The codominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flavonoid substances. Example: Gossypium hirsutum and Gossypium sturtianum, their F₁ hybrid (amphiploid) was tested for seed proteins by electrophoresis. Both the parents have different banding patterns for their seed proteins. In hybrids, an additive banding pattern was noticed. Their hybrid shows the presence of both types of proteins similar to their parents.

The heterozygote genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes. The F₁ heterozygotes produce an F₂ progeny in phenotypic and genotypic ratios of 1: 2: 1.