Genetic inheritance explains how characteristics pass from parents to offspring through DNA, which contains instructions for growth, survival, and reproduction. Variations in these inherited traits are determined by different versions of genes called alleles. Individuals typically inherit two alleles for each gene, one from each parent, influencing how their genes are expressed.
The Basics of Codominance
Codominance describes a genetic phenomenon where two alleles for the same gene are fully and equally expressed in an organism. In a heterozygous individual (possessing two different alleles for a trait), both alleles contribute to the observable physical characteristic, or phenotype.
The resulting phenotype distinctly displays the characteristics of each allele simultaneously. Neither allele masks the other, nor do they blend. Both are present and visible. For instance, if an organism inherits alleles for two different colors, both colors will appear distinctly rather than mixing into a new color.
Codominance Versus Other Inheritance Patterns
Codominance differs from other inheritance patterns, such as complete and incomplete dominance. Understanding these differences clarifies how traits are expressed.
In complete dominance, one allele completely masks the effect of another allele in a heterozygous pairing. The dominant allele’s trait is fully expressed, while the recessive allele’s trait remains unexpressed. For example, if a plant has one allele for purple flowers and one for white, and purple is completely dominant, the flowers will be purple.
In contrast, incomplete dominance occurs when the phenotype of a heterozygous individual is an intermediate blend of the two homozygous phenotypes. Neither allele is fully dominant, leading to a new, combined phenotype that is different from either parent. An example is a cross between red and white flowers producing pink flowers. Unlike these, codominance is distinct because both alleles are expressed separately and distinctly, without blending or one masking the other, allowing both traits to appear.
Common Examples of Codominance
A prominent example of codominance in humans is the ABO blood group system. This system involves three common alleles: IA, IB, and i. The IA allele leads to the production of A antigens on red blood cells, while the IB allele results in B antigens. Individuals inheriting both the IA and IB alleles will have AB blood type, meaning both A and B antigens are present and fully expressed on their red blood cells. The ‘i’ allele, which produces no antigen, is recessive to both IA and IB.
Roan cattle coat color is another example of codominance. Roan cattle exhibit both red and white hairs simultaneously, not a blended pink or light red color. This occurs because the alleles for red coat color and white coat color are codominant. Each individual hair on a roan animal is either entirely red or entirely white, creating a speckled appearance. Similarly, chickens with erminette feather patterns show codominance, displaying both black and white speckles.