Co-dominance

“Co-dominance” is another important concept in the Principles of Inheritance and Variation in genetics. It describes a scenario where both alleles in a heterozygous individual are fully expressed, leading to a phenotype that simultaneously shows both traits without blending. Key aspects of co-dominance include:

1. Equal Expression of Alleles: In co-dominance, both alleles are equally expressed in the phenotype. This differs from incomplete dominance, where an intermediate phenotype is observed.

2. Classic Example: A well-known example is the ABO blood group system in humans. The A and B alleles are co-dominant. Individuals with genotype IAIB express both A and B antigens on their red blood cells.

3. Phenotypes Reflect Both Alleles: In co-dominance, the phenotype of a heterozygote clearly shows the effects of both alleles. For instance, in cattle, the Roan coat color is a result of co-dominance where both red and white hair is present.

4. Genotypic and Phenotypic Ratios: In co-dominance, the genotypic ratio often reflects the phenotypic ratio, as each genotype corresponds to a distinct phenotype.

5. No Dominance or Recessiveness: Neither allele is dominant or recessive in co-dominant relationships. Both alleles are fully and equally expressed.

6. Molecular Basis: Co-dominance can occur when each allele codes for a different protein, or different versions of a protein, and both are functional and contribute to the phenotype.

7. Heterozygous Advantage: In some cases, heterozygotes may have an advantage in certain environments, known as overdominance. A classic example is the sickle cell trait, where individuals with one sickle cell allele (HbS) and one normal allele (HbA) have resistance to malaria.

8. Understanding Genetic Complexity: Co-dominance helps in understanding the complexity of genetic inheritance and expression, illustrating that not all genetic interactions follow simple dominant-recessive patterns.

9. Implications in Medical Genetics: Co-dominance is important in understanding various genetic conditions and in medical scenarios like blood transfusion and organ transplantation, where matching of co-dominant traits like blood groups is crucial.