- explains inheritance resulting in two dominant alleles being expressed in the

**Codominance Explained: Expression of Two Dominant Alleles** In genetics, **codominance** is a mode of inheritance where two different alleles for a particular gene are both fully expressed in the phenotype of a heterozygous individual. Unlike simple dominance, where one allele masks the effect of another (with only the dominant allele being expressed), codominance allows both alleles to contribute equally to the organism's traits. ### Key Characteristics of Codominance: 1. **Simultaneous Expression**: Both alleles in a heterozygote are fully and simultaneously expressed without one being dominant over the other. 2. **Distinct Phenotypes**: The resulting phenotype shows traits of both alleles distinctly, rather than a blended or intermediate trait. 3. **Genetic Diversity**: Codominance increases phenotypic diversity within a population by allowing multiple traits to express simultaneously. ### **Examples of Codominance:** 1. **AB Blood Type in Humans**: - **Genes Involved**: The ABO blood group system has three alleles: IA, IB, and i. - **Inheritance Scenario**: When an individual inherits both IA and IB alleles (genotype IAIB), both are equally expressed. - **Resulting Phenotype**: The person has type AB blood, displaying both A and B antigens on the surface of their red blood cells. 2. **Roan Coat Color in Cattle**: - **Genes Involved**: Coat color in cattle can exhibit codominance. - **Inheritance Scenario**: If a cow inherits one allele for red coat color (R) and one for white coat color (W), both alleles are expressed. - **Resulting Phenotype**: The cow has a roan coat, characterized by a mixture of red and white hairs. 3. **Cecropia Moth Wing Patterns**: - **Genes Involved**: Certain genes control the color patterns on the wings of Cecropia moths. - **Inheritance Scenario**: Heterozygous moths inherit one allele for a red pattern and one for a white pattern. - **Resulting Phenotype**: Both red and white patterns are visible on the wings simultaneously. ### **Genetic Representation:** Consider a gene with two alleles, **A** and **B**, both of which are dominant. - **AA**: The individual exhibits trait A. - **AB**: The individual exhibits both traits A and B (codominance). - **BB**: The individual exhibits trait B. ### **Punnett Square Example:** Let's examine a cross between two heterozygous individuals (AB x AB): | | A | B | |-----|----|----| | **A** | AA | AB | | **B** | AB | BB | **Genotypic Ratio**: - AA: 1 - AB: 2 - BB: 1 **Phenotypic Ratio**: - Trait A only: 25% - Both traits A and B: 50% - Trait B only: 25% ### **Distinguishing Codominance from Other Inheritance Patterns:** - **Complete Dominance**: One allele completely masks the other in the phenotype (e.g., AA and Aa appear the same, different from aa). - **Incomplete Dominance**: Heterozygotes display an intermediate phenotype (e.g., red and white flowers producing pink flowers). - **Codominance**: Both alleles are fully and distinctively expressed without blending (e.g., AB blood type). ### **Significance of Codominance:** Codominance plays a crucial role in creating genetic diversity and can have important implications in areas such as: - **Medicine**: Understanding blood type inheritance for safe blood transfusions. - **Agriculture**: Breeding livestock with desirable traits by maintaining multiple alleles. - **Evolution**: Maintaining genetic variation within populations, which can be advantageous for survival. ### **Summary:** Codominance is a fascinating genetic phenomenon where two dominant alleles are both fully expressed in the phenotype of an organism. This results in distinct and simultaneous traits being visible, contributing to the richness of genetic diversity observed in various species.