Understanding Codominance
Codominance is a pattern of inheritance where two different alleles for a single gene are both fully and equally expressed in a heterozygous individual, without blending. Instead of one allele masking the other (complete dominance) or producing an intermediate phenotype (incomplete dominance), both alleles contribute distinctly to the phenotype, and both traits are visible.
Key Principles of Codominance
In codominance, neither allele is recessive nor completely dominant over the other. Consequently, a heterozygous individual carrying these two codominant alleles will display characteristics associated with both, not a compromise or an 'in-between' trait. This phenomenon illustrates the direct and independent expression of each allele's genetic information at the phenotypic level.
A Practical Example: ABO Blood Group System
The classic example of codominance in humans is the ABO blood group system. The gene responsible for blood type has three alleles: Iᴬ, Iᴮ, and i. Here, Iᴬ and Iᴮ are codominant, while the 'i' allele is recessive. An individual with the genotype IᴬIᴮ will have blood type AB, meaning both A and B antigens are present on the surface of their red blood cells, fully expressing both alleles simultaneously and distinctly.
Importance and Applications
Codominance is crucial for understanding the full spectrum of genetic diversity and how certain complex traits are inherited. It helps explain patterns that deviate from simple Mendelian genetics, providing insights into various biological processes. Its practical applications include blood typing in forensic science, accurate paternity testing, and ensuring safe blood transfusions by recognizing the presence of multiple antigens.