How does codominance differ from simple dominance?

Codominance differs from simple dominance as both alleles are fully expressed in the phenotype, rather than one being masked.

In simple dominance, one allele is dominant over the other. This means that if an organism has two different alleles for a trait, the dominant allele will be the one that is expressed in the phenotype. The other allele, which is recessive, will be masked and will not be expressed unless the organism has two copies of it. For example, in pea plants, the allele for purple flowers is dominant over the allele for white flowers. So, a plant with one allele for purple flowers and one allele for white flowers will have purple flowers because the purple allele is dominant.

On the other hand, codominance is a form of dominance wherein the alleles of a gene pair in a heterozygote are fully expressed. This results in offspring with a phenotype that is neither dominant nor recessive. A classic example of codominance is the ABO blood group system in humans. In this system, the A and B alleles are codominant, meaning that if an individual inherits an A allele from one parent and a B allele from the other, they will have type AB blood.

In summary, the key difference between simple dominance and codominance lies in the expression of the alleles. In simple dominance, one allele masks the other, resulting in the expression of only the dominant trait. In contrast, in codominance, both alleles are fully expressed, leading to a phenotype that shows both traits.